Payment allocation becomes critical when a customer has multiple open invoices and sends a single payment that doesn't match any of them exactly. If a customer owes invoices of $3,200, $1,800, and $950 and sends $4,000, how should that $4,000 be applied? Close the $3,200 and put $800 toward the $1,800? Close the $1,800 and $950 with $1,250 toward the $3,200? The answer affects which invoices stay open, which ones age further, and which ones trigger collection activity.

Most accounting systems let you specify allocation rules: oldest-first, specific-invoice, or proportional. Oldest-first is the most common in collections because it closes the most-delinquent invoices first, stopping them from aging further and reducing legal risk. Customers sometimes specify which invoice they intend to pay; honor that instruction and apply the remainder to the next-oldest open invoice. Misapplied payments are a common source of customer disputes ("I paid that invoice") where the payment actually landed on a different invoice in your system.

Clear payment allocation rules protect you in two ways. They make sure you always know the exact balance due on each invoice, so collection contacts reference accurate figures. And they prevent customers from claiming an invoice was paid when the payment actually went somewhere else. Document your allocation policy and communicate it to customers who ask: "We apply payments to the oldest open invoice first unless you specify otherwise in writing."