Start with the evidence
Gather the remittance explanation and the original invoice. Compare references and currency before deciding that the difference belongs to this document.
Work through the example
An example USD480 receipt against USD500 leaves USD20 unresolved. Do not label it as a discount or fee without supporting evidence; those explanations have different source requirements.
Make the next step clear
Record the exact question and retain the response. Keep an open difference visible until the responsible reviewer approves the explanation and any resulting adjustment.
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.