External services - IV before GR
we need to deactivate the GR-Based Invoice Verification (GR-bsd IV) in a purchase order with External Services because we need to do IV before GR.
The system doesn't allows to uncheck this field and returns this message: Service-based invoice verif. requires GR "NO" or GR-based invoice verif.
With a ABAP exit we maked this field uncheck and we did an IV BEFORE the GR. But the GR take the gross value instead of net value.
How to solve our problem?