Best practice for handling warranties in an advance exchange scenario

All,

We are in the process of setting up Depot Repair for our organization. Most of our business will be handled with advance exchange scenario. My question is how did you guys handle the correct accounting for warranties?

Example:

1) Item 001 with S/N AB is shipped to the customer as a replacement (accounting: COGS)

2) Item 001 with S/N XY is being returned by the customer (without credit)

It could take several weeks to find out if the damaged product is covered by warranty. How can we assure that the costs for the shipped product go against warranty and the customer is not forced to pay the invoice?