In SO we have two line item i.e first line item having item category TAN & second one is TAS (service W/O delivery). For those we are using two different billing type ZSRP & ZGRC. So client requirement if any user is trying to make first line item billing,system should check second line item is billed completely then only it allow to user to create billing document in VF01. Otherwise it shouldn't allow.Please guide me how to achieve this ? Valuable answer must be rewarded.