If you have a payment on account, you can apply it to an outstanding invoice balance. To do this, you need to allocate the two transactions together.

1. Go to Contacts, and then click the applicable contact.
2. From the Manage menu, select Account Allocation.
3. Select the check box for the payment on account and the check box for the invoice to which you are applying the payment.
4. Make sure that zero (0.00) appears in the Left to Allocate field. The amount left to allocate must be zero in order to save the allocation.
5. Click Save.

When transactions are allocated together for a contact the allocation date will be the date of the most recent transaction. For example, if on 6/28/2018, you allocate and invoice dated 6/9/2018, a credit note dated 6/20/2018, and a payment dated 6/11/2018, the allocation date will be 6/20/2018.