Restrict material groups at PR/PO creation for items w/o material master

Dear SAP experts

I have one question related to restricting material groups which are possible for selection during purchase requisition or purchase order creation.

The material group is created and several material master records are linked to it. Now the requirement is that when users are creating either purchase requisitions or purchase orders it shouldn't be possible to select that specific material group>

But this should be valid only if PR or PO are being created for items without material master record. If PR/PO is created for item with material master record then the restriction should not take place.

What would be the nicest solution to achieve that?

Thank you all in advance for your help.

Regards,

Miha