Hello,
There are couple of options.
1. If you just wanna go without any change in the pricing procedure or records, you can simply use reason for rejection codes on the line items of the sales document after DP90. You can configure a new rejection code for that warranty case.
Well, incase if you running a batch job to create such sales documents then you can go with user exit to add rejection code automatically on your debit request. Reason code controls whether billing in required with zero value or not at all.
2. Accounting indicator on the service order
When you notice the machine is on warranty, you can change the accounting indicator accordingly say 02(free to customer)or you own code say Z1(under warranty).
Based on the accounting indicator, we have standard discount condition type KBM1 where you can maintain the condition record with 100-% discount. The moment you perform DP90, system gives the price and 100% discount will be applied and a zero value invoice can be produced and printed.
note:- In this case, you need to make sure this condition type exists in your pricing procedure otherwise use the suitable condition type that you use in your pricing procedure for discount and add the key combination with a condition table 140.