Change standard fields from PO - User Exit?

Hi All,

I need to clear two flags(REPOS/WEBRE) while creating PO based on particular order type and Item category. So can anyone tell me how can I clear or change standard fields from PO (during save), do we got any exits. This flag as to be cleared while using transaction ME21N/ME21/ME22N/ME22 or even when PO created/changed using BAPI’s. Thanks in advance.

Kind Regards,

Trim