Application & Removal of Billing Block on sales document/credit memo request

Hi all,

1. A secial billing block e.g. ZZ that gets put automatically when the credit memo request is saved. I see this can be possibly done with MV45AFZZ - USEREXIT_FIELD_MODIFICATION

Now here this gets trickier:

1. This block can be removed only by specific users

2. The other users should be able to remove all other billing blocks if they exist, via VA02/V23

3. If anyone tries to change this ZZ to any other BB then the system should revert it back to ZZ on saving - in other words- this ZZ can only be blanked out and by these specific users only

Any way to achieve this? I know we have the authorization object V_VBAK_AAT for activity 43 that can help for  but how do we restrict it to the value ZZ for this purpose? Appreciate all help in designing this step wise?

Thank you very much.