Start with the evidence
Gather the invoice and evidence of the amount received, including its currency and reference. Check that the payment belongs to this invoice before linking it to an open balance.
Work through the example
An example invoice of USD900 with a confirmed USD350 payment leaves USD550 to explain. Keep the original USD900 visible so that the remaining amount is not mistaken for a revised invoice total.
Make the next step clear
Record the payment reference and the date checked. If a fee, credit or another adjustment also exists, review it separately rather than forcing the difference into a second payment that did not happen.
Keep the source context
An invoice record and a customer conversation provide different evidence. Keep original document references stable while adding dated notes about replies and related payments. These guides concern clear administration, not collection tactics or promises of automated money movement.