Hi Heinz
Do you refer to dynpro SAPMV13N 0301 field PRQNR? The error is the type WBBY number 036? If it's true, then it is ruled in the include MV13NF_BBY_CHECK_PRQNR, form bby_check_prqnr, but I don't know how to avoid the error with an implicit enhancement, because you could write coding in the end of subroutine, but the error message would be triggered before. Other important issue is the reply by SAP, because you can order a customer mod for x+ Euro. All is hard-coded (number of items, type of error).
Perhaps the solution is create new bonus buy (until the item 999).
I hope this helps you
Regards
Eduardo