Payment Receipt with Warranties

We are an appliance sales and service organization and we are trying to figure out how to handle the following transaction.  Any help would be appreciated.

We visit a customer who is under warranty.  We perform the service (which includes parts).  We then file this with the relevant warranty service for claim.  We receive payment for the labor portion of the warranty.

THe issue is that we also order and are billed for the parts.  However, when the warranty is processed, the warranty compay pays our vendor directly for the part.  We need to track the parts as they are part of inventory, but are struggling to figure out how to do this when we get "paid" for the warranty.  We will have a credit issued to the parts vendor for our account.