If a subcontractor sends items back and the post-Statement over the purchase line has a faulty quantity posted to production order, a cancellation of the whole process is necassary. This should work automatically behind the process. Also a cancellation of its delivery note should arrange the same function, as Long as no invoice is posted yet or the procution order is not going on. Sometimes a fault can happend when subcontractor quantites will posted manually through the purchase order.

One further workarround should be to do the post Statement through the Warehouse receipt.