Wrong amount refunded to customer.

Hi,

in JDE, we have a credit memo of -100$ for customer refund. But during the payment, the payment was made for 120$. If I create a receipt of -120$ and apply to the CM of -100$, the system doesn't allow me to create it as it doesn't allow negative unapplied receipts. Now, we want to book the -20$ to the customers open amount. How can this be done?

Thank You.