AR Cash Receipt with Two Customers One a Positive Cash and One all Negative Cash

I have a check for payment that needs to be split between two customers.
One Customer has a positive amount of $20,000 for invoice payments.
2nd Customer has a Negative Amount of -$5,000. (CM on the customer)
Total Check is $15,000.
How do I apply the Negative Cash to the 2nd Customer. We do not have National Accounts Set Up.
Thanks

Write them off to common GL Account.

KR

You can apply the credit memo using the apply credit memo menu

Thank you. That seems logical. I appreciate your help.