Adjusting the value of the paid invoice will create an unapplied credit, rray.

The credit can be used on the remaining open invoice from the project. When you receive payment, you will see it as an Unapplied Payment under Credits section.

Make sure to mark the invoice and credits. As you can see on the screenshot below, the amount received shown is already less than the original amount because of the credit.

If you need additional help on this process, just leave a reply.