Step 1: Modify the installment payment method
The first step would be to "pay" the remainder of the invoice, using the Client credit balance, so that a debt is created in the member's profile.
To do this, click on the three buttons next to the planned payment > ''Take payment now'':
We convert the outstanding payments to Credit account:
Step 2: Adjust customer balance
As mentioned above, a debt will be created in the member's profile.
From the member's profile, click on "Adjust balance":
We add the missing amount, and click on "Do not generate receipt":
The missing invoice amount has already been cancelled!