Applying credit balances as orders payment

If a customer has an open credit balance, you can apply this credit balance as payment for either an existing order or a new order. You can apply all or part of a credit balance as payment. If you attempt to apply an amount in excess of the credit balance, an error message will be generated. Credit balances are applied as payment by using the *Tab command.

More:
Selecting open payments for orders: *Tab
Applying credit balances to other customers