Hi there,
You want some form of credit journal - you enter these in TASBooks program 2.2.2 (Sales > Enter/Change Journals > Enter/Change Customer Credit Notes/Credit Journals)
In that program there is an option for write-off - the nominal it will post the adjustment to depends on how your TASBooks is setup.
Post the journals then allocate them using program 2.5.1 (Sales > Receipts > Enter/Allocate Sales Ledger Receipts).
If you're not 100% sure then setup a Demonstation Company, recreate the scenario and try the various options to see which gets you what you want
Best / Doug.