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.

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To do this, click on the three buttons next to the planned payment > ''Take payment now'':
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We convert the outstanding payments to Credit account:
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​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":
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We add the missing amount, and click on "Do not generate receipt":

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The missing invoice amount has already been cancelled!