Service Agreement partial credit for subline process question

I am not sure how to proceed with doing a credit on a subline of a Service Contract. An example is a Service Agreement that is Active (start date 01-DEC-2010 for one year) and for a contract line there is one subline of level Item for quantity 15 priced at $29 each so the total for the subline and contract line that was billed was $435. We agreed to credit the customer for a quantity of 5 units (for $145). Since the contract was already billed, we are hoping to only create a credit memo for $145, but I am