Chargebacks in AR

We want to apply chargeback claims.  We like to manually enter the receipt chargback in the "Apply To" field  by just applying the claim number and of course you can't because it wont allow you to enter a number unless its part of the List of Values.  I am trying to find other alternatives of applying receipt chargeback claims.  I know there is the Trade Management Module but we do not have the budget for the licencing. I would like hear your suggestion, customer send a check for $1,200.  The check is to apply to invoices for $400, $500, $600,