How do I add additional payment to a customer's debit?

Easy!

In the Payments tab of the customer's page, select the amount of the debit you wish to add the payment.
In the Debit Details pop-up window, click the Add Additional Payment link.
In the Additional Payment field, enter the amount.
Click the Apply (tick) button.
In the Confirmation pop-up window, enter the reason for the extra payment.
Click OK.

Note: You cannot add additional payment to a booking. You can only modify the amount for each payment item.