How do we write off a credit?

We have situtation where we enter a prepayment, and for some reason we have to wrte off a part of the prepyament.

here is the example.

             We enter a prepayment for $10 and for customer A

                  Now the customer balance will show as $-10

             Now for some reason we want to write off a $1 so the customer balance will be $-9

            I have tried maintenance worksheet but was not able to do a write off it give some keep the signs message and other messages.

           Just wondering how we can do a write off on a credit.

Any help will be appriciated.