It would be great to have functions:
- conversion of the customer's payment into a prepayment for the Remaining amount
- conversion of the vendor's payment into a prepayment for the Remaining amount
- conversion of the customer's prepayment into a payment for the Remaining amount
- conversion of the vendor's prepayment into a payment for the Remaining amount

In Russian functionality we have only Return Prepayment function for the full prepayment amount