Material group in purchase order can be changed even if follow-on documents are created

[hubert_haist2]

In ME22N the material group can be changed even if the GR is done and an invoice is posted. I can't change the field selection key so that the field can't be changed in ME22N because the order is only set on hold the material group often needs to be changed. What can I do to make this field not editable anymore if follow-on documents are created?

We don't create orders with material master, only text orders.

Thank you

[JL23]

if SAP has nothing in standard then you have to do your validations in BAdI ME_PROCESS_PO_CUST