When your client makes an extra payment or payment received more than the invoiced amount, the payment allocated to invoice and the remaining payment will appear as unallocated credits (little green note) next to the contact under invoices section.

Allocating the credit amount to the invoices:

1. Go to billing/invoices

2. Search for the contact, click on the credit (green little note) 

3. Click on the unallocated credits

4. This will take you to the payment, click on Edit. 

5. You will be able to see the unallocated amount (credit) and outstanding amounts in red.

6. Now click on the plus symbol in red at the bottom (under Remaining). This will allocate the credit amount the outstanding invoice and click on Save.