editing/applying online payments

editing/applying online payments

We have customers who pay part or all of an invoice and then cancel their service and want the payment as a credit to future work.

It would be ideal if we could make it an excess payment and then void the invoice.  I can't make it an excess payment because i can't "edit an online payment amount".  I don't want to edit the amount, just where it is allocated.
I have been doing various work arounds, but honestly it feels like something zoho books should be able to fix for us.