A receipt records money in: from this customer, on this date, into this bank or cash account, by this method. Its second job is allocation — saying which invoices the money settles. The form suggests allocations against the customer's open invoices; you adjust them to match what the customer actually paid for. Whatever is left unapplied is not lost: it stands as credit on the customer's account, ready to apply to the next invoice.
Two special cases. A down payment for an order is a receipt taken against a specific order before its invoice exists; the money is held as a customer deposit — a liability of yours, not yet revenue — and is applied automatically when that order's invoice posts. If the deposit is larger than that invoice — the customer paid 10,000 against an order invoiced at 7,525 — the leftover stays held as a deposit, and it is not stranded: open the posted receipt and use Apply deposit to move some or all of what remains onto another posted invoice of the same customer, in the same currency. The application posts openly (deposit relieved, the invoice settled, any exchange difference between the two documents' frozen rates recognised as a realised gain or loss), the invoice's balance falls by the amount applied, and voiding that invoice later hands the money back to the deposit, exactly as a posting-time application unwinds. The customer's record shows the total still held on its Deposits Held card, and the receipts list's Unapplied column reads a down payment's deposit remainder, so held money is always visible somewhere. Refunding a deposit back to the customer is not yet a built action — for now, that money leaves only by being applied to an invoice. And when a sale came through a marketplace that deducts its cut before paying you, the receipt's marketplace fee field records what was deducted, openly, as a fee — never silently netted out of the amount.
Where the receipt's currency differs from your books' currency, the difference between the invoice's frozen rate and the rate at settlement is recognised openly as an exchange gain or loss at the moment the money lands — the multicurrency law of 1.11 applied to cash.