BADI / User Exits triggering after clicking SAVE button in ME21N

Hello Sir/Madam,

I have gone through the posted links to find list of user exits and finally understood EXIT_SAPMM06E_017 / BADI ME_PROCESS_PO_CUST triggers after clicking on SAVE button on ME21N screen. But , when I put break-point in either of the cases it triggers even before saving. Hence please if anyone knows the exit/ badi which triggers after SAVE please help.

I have to check if Tax Code (EKPO-MWSKZ) is maintained against a material, if not maintained throw error message to user before saving a PO.

Thank You.