Multiple Prepayments against Balanced Invoice

Acumatica does not allow us to apply a 2nd Prepayment against a Balanced Invoice if a Prepayment  already transferred to the Invoice from the Sales Order.

Here is our process:

1. Create SO
2. Apply Prepayment of 10% to SO
3. Create Balanced Invoice (do not release, we have to wait until our customer picks up the product to release the invoice) - Acumatica automatically pulls the SO Prepayment over to be applied to the Invoice.
4. When customer makes final payment prior to picking up the product, we want to apply a 2nd Prepayment against the Balanced Invoice - Acumatica does not allow this (I found a quirky workaround that if you temporarily unapply the SO Prepayment from the Invoice, apply the 2nd Prepayment, then reapply the SO Prepayment, it will allow you to apply both Prepayments).  The reason we use a Prepayment for the final payment prior to pickup is because we have situations where a customer makes a final payment but doesn’t pick up until the next month.  If we would use a ‘Payment’ type transaction, it creates negative A/R  which our CFO does not want to see on the balance sheet - he wants to see the dollars in a liability account (and we have Prepayments set to post to a liability account).

Any thoughts on how to be able to apply a 2nd Prepayment against a Balanced invoice?