Start with the evidence
Compare the received amount with the intended invoice and check for other documents in the remittance details. Confirm currency before calculating a difference.
Work through the example
A USD550 receipt is linked provisionally to a USD500 invoice, leaving USD50 to explain. That could require further context; the arithmetic alone does not authorize a refund or another allocation.
Make the next step clear
Keep the unresolved amount visible and assign the question to the appropriate person. Preserve the original receipt while the explanation is confirmed rather than changing its value to match the invoice.
Keep the source context
Use confirmed source records and keep their dates, references and currencies visible. A payment request is not a receipt, a provider status is not necessarily a bank entry, and a net settlement may combine several components.