Branch Out - Re: advance payment in JDE1

We are processing prepayments through AP. Our issue is that we make a prepayment for the whole amount for a service (say $100,000) and then the service is done in sections so we get partial invoices for each section (say $15,000). We create a voucher for the $15,000. When we try to apply it against the negative $100,000 we receive an error that the amounts do not match.

Is there a way of being able to apply the partial amount of $15,000 against the $100,000 prepayment leaving only $85,000 open?

Thanks for any help,

Charles