A Philippine Legal Article

An accidental bank transfer in the Philippines is not merely a banking inconvenience. It can produce a chain of legal consequences involving civil law, banking practice, electronic payment systems, quasi-contract, unjust enrichment, mistake in payment, possible criminal exposure in some situations, and court remedies when voluntary return fails.

The core legal problem is simple: money is sent to the wrong person, the wrong account, or in the wrong amount. The legal response, however, depends on how the transfer happened, who received it, whether the recipient knew or should have known of the mistake, whether the funds remain intact, whether the bank itself was at fault, and what evidence exists.

This article explains the Philippine legal framework as comprehensively as possible.

I. What is an accidental bank transfer?

An accidental bank transfer is any transfer of funds made without a valid intent to transfer that exact amount to that exact recipient under that exact circumstance.

It commonly happens in these forms:

transfer to the wrong account number
transfer to the correct account number but wrong intended recipient
transfer of the wrong amount
duplicate transfer
transfer caused by typographical error
transfer to a recycled or reassigned account
transfer induced by fraud but authorized by mistake
transfer caused by system error, auto-fill, QR confusion, or mobile banking interface mistake
transfer made by a bank through posting or processing error

Not all accidental transfers are legally identical. Some are primarily customer mistakes. Others are bank errors. Some are mixed cases. The remedy may differ accordingly.

II. The governing Philippine legal principles

In the Philippines, the main legal principles behind accidental transfer cases come from:

the Civil Code on obligations and quasi-contracts
the doctrine against unjust enrichment
the rule on payment by mistake
the law on damages
the rules on contracts and agency, where relevant
the Electronic Commerce Act framework for electronic transactions
banking regulations and internal bank dispute procedures
payment network rules for electronic fund transfers
court procedures for recovery of money
in some cases, the Revised Penal Code or special penal statutes if the recipient fraudulently keeps or dissipates funds with bad faith

The deepest civil-law anchor is the rule that no person should unjustly enrich himself at the expense of another. If a recipient gets money through mistake and has no lawful right to keep it, the law generally treats that person as obliged to return it.

III. The central civil-law basis: solution indebiti

The most important legal doctrine in accidental transfer cases is solutio indebiti or solution indebiti.

This applies when:

something is delivered through mistake, and
there is no right to demand it.

In plain terms, if money was transferred by mistake and the recipient had no legal right to receive it, the recipient is generally obliged to return it.

This doctrine is a form of quasi-contract. It does not depend on consent between sender and recipient. The law itself creates the obligation to return what was unduly received.

This matters because in many accidental transfer cases:

there is no contract between sender and accidental recipient
there was no donation
there was no debt
there was no intent to benefit the recipient

So even without a contract, the law provides a basis for recovery.

IV. Unjust enrichment

Closely related is the doctrine of unjust enrichment.

A person cannot legally retain a benefit received at another’s expense when there is no just or legal ground for keeping it. In bank transfer mistakes, the enrichment is the receipt of funds; the impoverishment is the loss suffered by the sender.

This principle often supports the practical moral intuition behind these cases: “The money is not yours.” Philippine civil law gives that intuition legal force.

V. The first major distinction: sender error versus bank error

This distinction is crucial.

A. Sender error

This happens when the account owner makes the mistake, such as:

entering the wrong account number
selecting the wrong saved recipient
encoding the wrong amount
authorizing the wrong transfer
sending twice by accident

In these cases, the sender generally has a claim against the accidental recipient, and sometimes limited remedies through the bank’s dispute process, but the bank may argue it merely followed the sender’s instructions.

B. Bank error

This happens when the bank itself causes the mistaken transfer or wrongful credit, such as:

double posting
misrouting funds
unauthorized debit or credit from processing fault
clerical error in branch operations
system malfunction
erroneous reversal or failed reversal

In these cases, the bank may bear primary responsibility to correct the error, and the injured party may have claims against the bank for restoration, correction, and sometimes damages.

C. Mixed cases

Sometimes both are involved, such as where a sender made a minor encoding error but the bank’s system failed to detect an invalid account-recipient mismatch that it would ordinarily catch. Liability then becomes more fact-specific.

VI. The second major distinction: authorized transfer versus unauthorized transfer

An accidental transfer is not always the same as an unauthorized transfer.

Authorized but mistaken transfer

The account holder personally initiated the transfer, but it was done by mistake. This is usually a payment by mistake problem.

Unauthorized transfer

The account holder did not consent to the transfer at all. This may involve:

hacking
phishing
account takeover
insider misconduct
card compromise
system breach

This is legally different. Unauthorized transfer cases focus more heavily on:

bank security obligations
authentication failure
negligence
fraud investigation
possible criminal complaints

A transfer can be both mistaken and fraud-related, but the legal theories shift depending on whether the sender truly authorized the act.

VII. Immediate practical reality: banks usually cannot simply grab back the money

Many people assume that once an accidental transfer is reported, the bank can immediately reverse it. That is often not how it works.

In practice, a bank usually cannot freely and unilaterally debit another depositor’s account without legal or contractual basis, especially once the funds have reached and been credited to the recipient account. Banks are bound by deposit relationships, privacy rules, due process concerns, and operational rules.

What banks often can do is:

log the complaint
trace the transaction
attempt a recall or reversal request within the payment network
contact the receiving bank
request the recipient’s consent to return funds
place temporary controls if allowed by law, regulation, network rules, or internal fraud protocols
investigate whether the transfer was due to bank error or unauthorized access

But where the recipient refuses and the transfer was actually completed into the recipient’s account, recovery may become a matter of formal dispute resolution or court action.

VIII. Internal bank complaint is the first line of remedy

The first remedy is almost always to report the matter to the sending bank immediately.

The complaint should include:

name of account owner
date and time of transfer
amount
source account
destination account details
transaction reference number
screenshots or proof of transfer
explanation of the mistake
request for immediate recall, trace, hold, or coordination with receiving bank

Speed matters greatly. If the recipient has not yet withdrawn or transferred out the funds, the chance of recovery is much higher.

Delays can make recovery harder because:

funds may be withdrawn
the recipient may dissipate the money
records become more difficult to secure
payment network reversal windows may lapse
the receiving bank may have limited basis for action absent a timely dispute

IX. Sending bank versus receiving bank

Accidental transfer cases often involve at least two banks:

the sending bank
the receiving bank

The sending bank usually deals directly with the sender. The receiving bank usually holds the account of the accidental recipient.

The sender’s relationship is usually contractual with the sending bank, not the receiving bank. That means the sender’s first formal complaint is usually against or through the sending bank.

The receiving bank, however, may become critical because it controls the destination account and may be the one in a position to contact its depositor, investigate movements, or respond to a lawful request, subpoena, or court order.

X. The accidental recipient’s legal duty

The accidental recipient generally has no right to keep money sent by mistake.

Once the recipient knows, or should reasonably know, that the transfer was accidental, the legal duty to return strengthens. If the person keeps, hides, spends, or transfers the money despite clear notice of mistake, the risk of legal liability increases substantially.

The recipient’s possible arguments may include:

the money was actually owed
the transfer was intended
there was a valid underlying transaction
the recipient changed position in good faith
the recipient no longer has the funds
identity of sender or transaction is unclear

But where there is no legal basis for receipt, the ordinary rule is return.

XI. Is the recipient automatically criminally liable for keeping the money?

Not automatically.

This is important. Mere receipt of a mistaken transfer is not automatically a crime. Civil liability to return the money may exist even where criminal liability does not.

Criminal exposure becomes more plausible when there is bad faith, such as:

refusal to return after clear notice
concealment of identity
quick withdrawal after learning of the mistake
transfer of funds to avoid recovery
use of deception to induce or preserve the mistaken credit
coordinated fraud with others
falsification, misrepresentation, or intentional appropriation

Not every mistaken payment case is estafa. Not every refusal to return becomes criminal. The exact facts matter, and prosecutors will look for more than simple receipt.

XII. Civil liability is usually the surest legal basis

The strongest and most consistent theory in accidental transfer cases is civil recovery.

A sender may pursue return of the money through:

demand letter
bank-assisted complaint process
mediation or settlement
civil action for sum of money
action based on solutio indebiti
action based on unjust enrichment
claim for damages where bad faith or negligence is proven

This is often more doctrinally stable than trying to force every case into a criminal theory.

XIII. Demand letter

A formal demand letter is often the next step when voluntary bank-assisted return fails.

It typically states:

the fact of mistaken transfer
date, time, amount, and reference number
lack of legal basis for the recipient’s retention
demand for return within a fixed reasonable period
notice that civil and, where warranted, criminal remedies may follow
possible claim for interest, attorney’s fees, and damages

The demand letter matters for several reasons:

it clearly establishes notice
it helps prove bad faith after refusal
it may trigger settlement
it can affect claims for damages and interest
it strengthens the evidentiary record

In some cases the sender may not know the recipient’s full identity. Then the bank process, subpoena, or later court action may be needed to identify the proper defendant.

XIV. Can banks disclose the recipient’s identity?

This is often difficult.

Philippine banking practice places strong emphasis on confidentiality. Banks typically do not casually disclose account-holder details to private complainants without lawful basis. Even when a sender can prove mistaken transfer, the bank may be cautious about releasing the recipient’s full identity absent:

recipient consent
lawful court process
regulatory basis
criminal investigation request
other legally sufficient ground

This means a sender may know only:

partial account number
recipient name as shown in the app
bank name
reference number

Where the recipient resists and identity is incomplete, formal legal process may be necessary.

XV. Can the recipient say, “I already spent it”?

That is not usually a complete defense.

If the money was received by mistake and there was no right to it, the duty to restore generally remains. The fact that the money was spent does not ordinarily erase the obligation.

However, it can affect:

collectability
urgency of relief
possible defenses about good faith
damages
choice of remedy
practicality of execution

A recipient who spends money after learning it was accidental is in a much worse legal position than one who innocently received it and promptly cooperated.

XVI. The bank’s own rights when it mistakenly credits an account

Banks themselves may also become victims of mistaken transfers or mistaken credits.

If a bank erroneously credits a depositor’s account, the bank generally has a legal basis to correct the error, subject to law, contract, and due process. The recipient of a mistaken credit is not entitled to insist on keeping money that never legally belonged to him.

But banks must still act carefully. Wrongful freezing, wrongful debiting, or sloppy error correction can expose the bank to separate liability.

So even when the bank is substantively right, its method of correction must still be lawful.

XVII. Wrong amount, duplicate amount, and partial return cases

Not all mistaken transfer disputes involve a completely wrong recipient.

A. Wrong amount

If the transfer was intended but the amount was excessive, the excess may be recoverable as undue payment.

B. Duplicate transfer

If the same obligation was paid twice by mistake, the second payment may generally be recovered.

C. Partial return

If the recipient returns only part of the funds, the balance remains recoverable.

D. Partial entitlement

If the recipient was entitled to some amount but received more than was due, only the excess is ordinarily subject to return.

These are still forms of mistaken payment, but the accounting becomes more detailed.

XVIII. Accidental transfer to a closed, dormant, invalid, or mismatched account

The result depends on the payment rail and bank system involved.

Possible outcomes include:

transaction rejected and auto-reversed
funds placed in suspense or pending status
funds credited if the account number controls the transaction
mismatch flagged by system and transfer blocked
account no longer active, requiring manual reconciliation

A sender should not assume that a wrong-name entry protects the transfer if the account number is accepted by the system. In many electronic transfers, the account identifier may be the controlling field.

This is why transaction proof and time stamps are critical.

XIX. Electronic wallets and non-bank channels

Many mistaken transfers now happen through:

e-wallets
digital banks
mobile apps
QR payments
InstaPay and PESONet channels
online merchant settlement flows

The same core civil-law principles still apply: money sent by mistake without legal basis is generally returnable.

But digital channels create special problems:

faster dissipation of funds
limited customer service windows
platform-based rather than branch-based complaints
screen-name ambiguity
layered intermediaries
app terms and conditions affecting dispute handling

The remedy remains legally similar, but the operational path may be more technical and time-sensitive.

XX. Payment network timing matters

In practical terms, accidental transfer cases are highly time-sensitive because electronic fund transfer systems differ in speed and reversibility.

Some transfers are near real-time, meaning the recipient can withdraw almost immediately. Others settle in batch or allow a longer recall window. The legal claim may remain, but the practical chance of easy retrieval shrinks rapidly once settlement is complete and funds are moved.

That is why the best phrase in these cases is not merely “I need a refund,” but “Please immediately trace, recall, and coordinate with the receiving institution because this was a mistaken transfer.”

XXI. Is the sender barred because the mistake was his own fault?

Generally, no.

A sender’s negligence in encoding the wrong account does not automatically legalize the recipient’s retention of money. The sender’s mistake may explain how the transfer happened, but it does not create a legal right in the recipient to keep the funds.

However, sender negligence may matter in:

claims against the bank
allocation of blame in damages
factual sympathy
contractual defenses based on agreed platform terms
proof issues where the sender’s story is unclear

The sender’s carelessness may weaken some claims against the bank, but it usually does not destroy the core claim against the accidental recipient.

XXII. Claims against the bank

Whether the sender can recover from the bank itself depends on the facts.

A bank may be exposed where:

it processed a transfer contrary to instructions
it duplicated a transfer
it mishandled reversal
it committed posting errors
it failed to observe required security measures
it negligently enabled unauthorized transfer
it breached its contractual obligations
it acted with bad faith or gross negligence in handling the complaint

But where the bank accurately followed the sender’s own authorized instructions, the bank may argue it is not liable for the sender’s encoding mistake and that the sender’s remedy lies against the recipient.

Bank liability is therefore highly fact-dependent.

XXIII. Damages

In Philippine law, accidental transfer disputes can lead to claims for damages, but not every case justifies them.

Possible damages theories include:

Actual or compensatory damages

For proven monetary loss directly caused by the mistake or wrongful refusal to return.

Interest

If money is wrongfully withheld after demand, interest may be claimed depending on the legal basis and court findings.

Moral damages

Usually require bad faith, fraud, or conduct causing recognized mental anguish under the law. They are not automatic.

Exemplary damages

Possible in cases of wanton, fraudulent, reckless, or oppressive conduct.

Attorney’s fees

May be awarded in the situations recognized by law, especially where the defendant’s bad faith forced litigation.

Damages against a bank usually require a more developed showing of negligence or bad faith. Damages against a recipient often depend on notice and refusal.

XXIV. Small claims court

Many accidental transfer disputes are fundamentally money recovery disputes. Depending on the amount and the current procedural limits in force, a sender may be able to use small claims procedure if the case qualifies.

This can be attractive because it is designed for simpler money claims and avoids some of the complexity of ordinary civil litigation. Whether it is the best route depends on:

amount involved
availability of recipient identity and address
evidence completeness
whether injunctive or discovery relief is needed
whether the bank must also be impleaded

For straightforward money recovery from an identifiable recipient, small claims may be practically useful.

XXV. Ordinary civil action

If the case is large, factually contested, or involves multiple parties, an ordinary civil action may be more appropriate.

This may seek:

recovery of a sum of money
declaration of obligation to return mistaken payment
damages
preliminary remedies where justified
orders directed at parties holding or tracing funds

Where the funds passed through multiple accounts or were mixed with fraud, more complex litigation may be necessary.

XXVI. Preliminary remedies

In serious cases, a claimant may consider provisional court remedies where legal requirements are met. These may become relevant if there is a real risk that the recipient will hide assets or frustrate recovery.

Such remedies are not automatic. Courts require strict grounds. But in cases involving quick dissipation or deception, early judicial relief may be strategically important.

XXVII. Barangay conciliation

If the parties fall within the coverage of barangay conciliation rules and live within the proper territorial relationship required by law, pre-litigation conciliation may be necessary before filing certain court actions.

Whether this applies depends on:

residence of parties
nature of the claim
amount
whether a corporation or bank is involved
specific procedural exemptions

This is not always applicable in banking cases, especially when institutions and cross-city parties are involved, but it should be checked.

XXVIII. Interest on the amount to be returned

If mistaken funds are wrongfully withheld after demand, interest may become recoverable depending on the nature of the obligation and the court’s findings.

The exact rate, reckoning point, and legal basis depend on current jurisprudential application and the structure of the claim. As a practical pleading matter, claimants typically allege interest from formal demand or from filing of the complaint, subject to court determination.

XXIX. Proof and evidence

The success of an accidental transfer claim depends heavily on proof.

Important evidence includes:

transaction confirmation
screenshot of transfer
SMS or email bank alerts
bank statement
reference number
account details used
chat logs or messages with recipient
complaint ticket with bank
formal written demand
proof of receipt of demand
bank replies
chronology of reporting
evidence that there was no debt or legal basis for payment

If the issue is bank error, then additional evidence may include:

branch records
reconciliation reports
audit logs
app logs
internal complaint results
authentication trail

Good documentation often determines whether the dispute settles or escalates.

XXX. Burden of explanation by the recipient

Once the sender presents strong evidence that a payment was made by mistake and that no obligation was owed, the recipient’s position becomes difficult unless he can show a lawful basis for keeping the money.

For example, the recipient may need to establish:

an actual loan repayment
payment for goods or services
prior obligation
authority to receive
settlement agreement
donation

Bare denial may not be enough if the transaction trail clearly shows mistaken remittance.

XXXI. Good faith versus bad faith recipient

This distinction is important for remedies and tone of liability.

Good faith recipient

A person who genuinely did not know of the mistake and cooperates once notified is still usually obliged to return the money, but exposure to damages or criminal issues is lower.

Bad faith recipient

A person who learns of the mistake and then hides, withdraws, lies, or refuses without basis is far more vulnerable to:

damages
attorney’s fees
stronger equitable condemnation
possibly criminal complaints if the facts support them

The obligation to return may exist in both cases. Bad faith mainly aggravates the consequences.

XXXII. What if the sender was paying the wrong person because of scam or impersonation?

This is a common modern variation.

If the sender intentionally sent money, but did so because of deception, impersonation, fake invoices, or social engineering, the case may involve both:

mistaken payment / unjust enrichment against the recipient, and
fraud-based claims against the wrongdoer

In such cases:

urgency is extreme
banks should be notified immediately
preservation of digital evidence is crucial
criminal complaint may become more viable
tracing may matter more than pure mistaken transfer doctrine

Where the recipient is part of the fraud, bad faith is easier to establish. Where the recipient is merely an account used by another, the case becomes more complex.

XXXIII. What if the transfer was made to a family member, employee, or known person by mistake?

These cases are often easier evidentially because identity is known. The legal doctrine remains the same, but proof and service of demand are easier. Informal settlement is more common, though emotional and relational conflict may complicate matters.

The sender should still document:

the error
the demand
the recipient’s response
any admission or refusal

Known identity makes litigation more straightforward.

XXXIV. What if the accidental recipient dies, disappears, or becomes insolvent?

The right to recover does not automatically disappear, but enforcement becomes harder.

Possible consequences include:

claim against the estate, if applicable
suit against known recipient while alive if already filed
difficulty executing judgment
tracing issues if funds were dissipated
strategic focus on other liable parties, if any, including the bank where warranted

A strong legal claim is not the same as an easy recovery.

XXXV. Prescription

Actions to recover money paid by mistake do not remain enforceable forever. Prescription rules matter. The exact period depends on the nature of the action as framed under Philippine law. Because mistaken transfer cases may be pleaded under quasi-contract, unjust enrichment, or related theories, prescription analysis should be handled carefully in actual litigation.

The practical lesson is simple: do not delay.

XXXVI. Can the sender offset the mistake against another debt?

Only in limited and fact-specific circumstances. If the accidental recipient separately owes the sender money, legal compensation or offset is not something to assume casually. The mistaken transfer still has to be analyzed on its own legal basis. Informal self-help setoff can create fresh disputes.

XXXVII. Can the bank freeze the recipient account?

Possibly in limited circumstances, but not automatically and not merely on demand by a private complainant.

A bank may act where there is:

clear internal error to correct
suspicious or fraudulent activity
lawful regulatory basis
court order
law enforcement request within legal bounds
network dispute process support
account agreement basis

But freezing someone’s funds without lawful basis can itself be problematic. This is why banks are often careful and procedural.

XXXVIII. Cross-border accidental transfers

If the mistaken transfer reached a foreign bank or came from abroad, the legal and practical complexity increases sharply.

Issues may include:

conflict of laws
foreign bank secrecy rules
intermediary institutions
currency conversion
SWIFT or remittance channels
foreign court process
delayed tracing

The same moral core remains, but enforcement may require international coordination and become significantly harder.

XXXIX. Employer payroll mistakes and corporate transfer mistakes

A special class of cases involves:

salary overpayments
duplicate payroll credits
vendor payment errors
treasury mistakes
mistaken corporate disbursements

These are still often recoverable as mistaken payments. However, labor law, company policy, employment relationships, accounting controls, and ongoing obligations may complicate how recovery is done. Employers must avoid unlawful wage deductions, and corporations should handle recovery in a procedurally sound way.

XL. How Philippine courts are likely to view the matter in principle

At the level of principle, a Philippine court is likely to ask:

Was money transferred?
Was the transfer mistaken?
Did the recipient have a legal right to the money?
Was there demand to return?
Did the recipient act in good faith or bad faith?
Did the bank commit error or negligence?
What losses were proven?
What remedy best restores the parties to their rightful positions?

The legal system generally aims to restore the sender where the transfer had no legal basis, without automatically punishing every recipient as a criminal.

XLI. Practical hierarchy of remedies

In actual Philippine practice, the remedies usually unfold in this order:

1. Immediate bank report

This is urgent and essential.

2. Formal written complaint and recall request

Create a record immediately.

3. Coordination with receiving bank / recipient

Voluntary return is the fastest solution.

4. Demand letter

This sharpens the legal position.

5. Regulatory or consumer complaint channels, where appropriate

Useful if bank handling is inadequate.

6. Civil action for recovery

Usually the strongest formal remedy.

7. Criminal complaint, if facts clearly show bad faith, fraud, or intentional appropriation

Only where the evidence supports it.

XLII. Common mistakes by senders

People often weaken their own case by:

waiting too long to report
relying only on hotline calls with no written complaint
failing to save screenshots
emotionally threatening the bank instead of stating facts
assuming the bank can always reverse immediately
not sending a formal demand
not preserving recipient messages
framing a civil mistake as a crime without enough evidence
deleting app logs or device messages
failing to identify whether the transfer was authorized or unauthorized

Clean documentation and legal precision matter.

XLIII. Common misconceptions

“The bank has to refund me because I made a mistake.”

Not always. If the bank merely followed your valid instruction, your direct claim may be against the recipient, not automatically against the bank.

“The recipient can keep it because I sent it voluntarily.”

No. Voluntary physical act of transfer is not the same as legal intent to pay that person.

“Once the money hits the account, it is legally theirs.”

No. Crediting does not create ownership where the transfer had no legal basis.

“Refusing to return automatically means estafa.”

Not automatically. Civil liability is easier to establish than criminal liability.

“If the recipient spent it, I lose my remedy.”

Not necessarily. The obligation to return generally remains.

“A screenshot is enough.”

Helpful, but stronger recovery usually needs the whole evidentiary trail.

XLIV. Special point on bad-faith silence

A recipient who notices a clearly anomalous deposit and says nothing may slide from good faith into bad faith depending on the circumstances.

For example, if a person with no expectation of payment suddenly receives a substantial transfer from an unknown sender and immediately drains the account after being notified, that conduct is legally and morally very difficult to defend.

The law does not favor strategic silence designed to preserve a windfall.

XLV. Relationship between equity and strict law

Accidental transfer disputes are a good example of law and equity pointing in the same direction.

Strict civil law says:

payment by mistake is recoverable
unjust enrichment is not allowed
one who receives what is not due must return it

Equity says:

nobody should profit from another’s obvious error

That is why accidental transfer cases often feel straightforward in principle, even if bank procedure and evidence make them difficult in practice.

XLVI. The best legal formulation of the sender’s claim

In Philippine terms, the cleanest legal formulation is often:

The funds were transferred by mistake, without any lawful obligation or cause, and are therefore recoverable under the rules on solutio indebiti and unjust enrichment, with damages if refusal or bad faith is shown.

That formulation usually captures the heart of the case.

XLVII. Final doctrinal summary

In the Philippines, an accidental bank transfer is usually governed by the law on payment by mistake, quasi-contract, and unjust enrichment. The accidental recipient generally has no right to keep funds received without legal basis. The sender’s first practical step is immediate bank reporting and recall efforts. If voluntary return fails, the sender’s strongest formal remedy is often a civil claim for recovery of the mistaken payment, supported by transaction records, written demand, and proof that no obligation existed.

Bank liability depends on whether the bank itself caused or mishandled the transfer. Recipient bad faith strengthens claims for damages and may, in some cases, support criminal proceedings, but criminal liability is not automatic. The decisive issues are who made the error, whether the transfer was authorized, whether the recipient had any lawful entitlement, how quickly the matter was reported, and what evidence exists.

In doctrinal terms, the rule is simple: money sent by mistake is not lawfully owned by the accidental recipient merely because it was credited to that person’s account. The law ordinarily requires its return.