User Exit for Price check during billing

Hello All-

Can some one please advise if there is any user exit, which will check if pricing is not maintained in VK11, then it should not process the billing document, we are having issue in Scheduling agreement, if a sale price is not maintained for any material, still system is creating invoices with 0 price, and this is only happening for Scheduling agreement.

If an user exit or validating can be added prior generating billing invoices, which will flag those Scheduling agreement then user can maintain the prices before generating the billing document.

Thanks for helping-

Marck