Go to the Payment on Accounts screen, and select the Customer. Enter the amount of Unapplied Cash to write off in the Discount Amount field as a negative number.
Enter a different account in the Disc Acct field if an account different from the default Discount G/L Account is preferred. Make sure no Invoices are checked in the Invoice grid below. Select Save.
A screen will appear stating The discount amount does not match the grid discount total. Do you wish to override discount amount? Select No.
Another screen will appear asking Do you wish to place the discount in Unapplied Cash? Choose Yes. The Unapplied Cash will be zeroed out.