Replacement in waranty period

Hi All...

I have a Problem and I need Your help on that ..

Problem.....1. Items are managed by serial Number

2.Customer(my) Provide waranty of 2 to its customer on its item..

3. he sales the item ...

4. If there is any problem in that item with in 2 years he replaced that item by newer one ..

5. He received the old item and then issue the new or old item on behalf of that ...

Problem is how i match the process as .... he replced the item at Zero cost ..... means he did not take any money and replces the item without any tax ................

thanks & regards

Rajnish