When you add a deduction to a payment, the system creates two postings when the payment is saved. One posting is recorded in the customer payments daybook and the second posting is recorded in the customer deductions daybook. A customer invoice of type Deduction is created when you save the payment. The customer deduction invoice uses the daybook linked to the bank GL account for the second posting (deductions daybook).

The system uses the Deduction Suspense system type account to transfer the balance from the payment posting to the deduction posting.