How to refund unused/remaining Prepayment Request in Project Contract Billing?

Trying to understand how to refund any unused or remaining balance on a prepayment request in Project Contract Billing.

This was raised in the following post but was never fully addressed:

To answer the questions in the initial response to that post, the scenario would be:

-Prepayment Request event type of $250,000 has been invoiced and paid (thus is in Accepted status and receipt has been applied in Receivables).

-Prepayment Application events totaling $200,000 have been invoiced and paid as lines on standard invoices.

-The scenario is that the entire prepayment amount did not end up being needed for that contract, so there is a $50,000 prepayment balance that remains open. There is no clear way to reduce the prepayment amount to match the final billing amount and refund that open balance back to the customer.