How do I match a lump-sum remittance to many open invoices?
Match against the remittance detail that comes with the payment — a BAI2 lockbox record, an EDI 820 advice, or a remittance email listing invoice numbers — rather than guessing how a lump sum splits. When that detail is missing, fall back to amount-combination matching, and treat any leftover as its own item to resolve, not force-fit into an invoice it doesn't match.
By Zuny Fester, Head of Operations and MarketingPart of the accounts receivable and collections guide.
| Primary matching source | Remittance detail attached to the payment — BAI2 lockbox record, EDI 820, or a remittance advice |
|---|---|
| BAI2 | A bank-defined flat-file format that carries invoice-level detail alongside lockbox deposits (Bank Administration Institute standard) |
| Fallback when detail is missing | Amount-combination matching — testing whether the lump sum equals the sum of a specific set of open invoices |
| What not to do | Splitting the lump sum evenly, or applying it oldest-invoice-first, without evidence that's actually how the customer intended it |
| Leftover balance | Treated as its own unresolved item (unapplied cash or a deduction), not forced onto an invoice it doesn't reconcile to |
Why guessing the split is worse than leaving it unapplied
A single wire or check covering a dozen open invoices doesn't come with an obvious answer for how it splits — and applying it oldest-invoice-first, or evenly across everything open, without evidence that's actually what the customer intended, can close out the wrong invoices while leaving the ones the customer actually meant to pay still open. A wrong match is arguably worse than no match: unapplied cash is visibly unresolved and gets investigated, while a wrong match looks resolved and quietly misstates which specific invoices are actually still owed.
What a BAI2 lockbox record actually provides
BAI2 is a bank-defined flat-file format, originally specified by the Bank Administration Institute, used to transmit deposit and transaction detail from a bank to a corporate treasury or AR system. When a customer pays through a lockbox, the bank's BAI2 file can carry lockbox remittance records listing the specific invoice numbers and amounts included in that deposit — turning a single lump-sum bank deposit back into its constituent invoice-level detail without anyone having to call the customer to ask.
What to do when remittance detail is missing or incomplete
Not every payment arrives with clean remittance detail — a wire with no accompanying advice, or a BAI2 record that lists an amount but not which invoices it covers. The fallback is amount-combination matching: testing whether the payment total equals the sum of some specific subset of that customer's open invoices. A lump sum that exactly equals three specific open invoices, and no other combination of open invoices on that account, is strong evidence for that match even without explicit remittance detail — though it's meaningfully weaker evidence than an actual remittance record naming the invoices directly, and should be treated with more scrutiny.
Why a leftover balance needs its own resolution, not a shortcut
A lump sum rarely divides evenly. Once the matched portion is applied, whatever's left over — too little to close the next invoice, or an amount with no combination that matches anything open — shouldn't be force-applied to the nearest invoice just to zero it out. That leftover is either a deduction (see the short-payment and deduction page) or genuinely unapplied cash, and it needs the same classification discipline as any other unmatched amount, not a shortcut that makes the reconciliation look cleaner than it actually is.
Next step
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Field mapping
A $27,650 lockbox deposit mapped line-by-line to four invoices, with a leftover
| BAI2 lockbox detail record | Invoice # | Amount applied | Result |
|---|---|---|---|
| Detail line 1 | INV-8801 | $9,200.00 | Matches invoice exactly — closed |
| Detail line 2 | INV-8815 | $6,450.00 | Matches invoice exactly — closed |
| Detail line 3 | INV-8822 | $11,000.00 | Matches invoice exactly — closed |
| Remaining deposit | None listed | $1,000.00 | No matching detail line — held as unapplied cash, not forced onto INV-8830 ($1,050) despite being close |
$26,650 of the $27,650 deposit matches explicit BAI2 detail lines and closes three invoices cleanly. The remaining $1,000 is close to, but not exactly, the $1,050 balance on INV-8830 — without an explicit remittance line naming it, applying that $1,000 to INV-8830 would be a guess, not a match. It's held as unapplied cash pending confirmation, rather than applied on a near-miss.
Frequently Asked Questions
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