How to apply an unpaid prepayment to a standard invoice?

Hi,

I would appreciate your help in finding a workaround on how to apply an unpaid prepayment to a standard invoice.

I know that the prepayment invoice should be paid and the status should be "Available" for this invoice, but we have a client that wants to apply an unpaid prepayment to a standard invoice even though the prepayment is not paid and the status for this invoice is "Unpaid".

He didn't actually pay the prepayment nor the invoice but for accounting purpose he wants to enter and create accounting for them in the system so they show as a liability for next year.