Happy New Year

I have a question about how to track my comeback/warranty parts and labor.
We are concerned that our profit reports are incorrect.
When a part is defective we have the customer come back in. We start a new RO and put the replacement part and labor on it.
We use a "warranty" coupon to 00.00 out the labor
We are charged for the part so we put the part on the ticket normally and then we 00.00 out the
"price" of the part.
Our labor is refunded with a credit from our vendor
The part is charged and then credited on our statement.
We are concerned that our reports for the parts coming into the shop and going out without a profit skew the profit reports and the labor profit reports will also be skewed.
How does the system handle the warranty parts? How is does the system handle the labor. How are others handling this problem.
BTW - I searched the Forum for how others are handling this problem but my search must not have been very good on the search of "warranty" or "comeback" - nothing on how to handle the paperwork side of the problem.
Thanks - Paula