USER EXIT for Purchase Order Save

Hi Experts,

I have requirement  to while creating PO and saving time need to some validate with some condition like EKKO-LIFNR NE T001W-LIFNR then message system should restrict with message 'Enter Purchase organisation'. So where i need to implement this logic and that logic should work for bachjob and BAPI time also. And how to display this message on screen and while creating though BAPI.

Anyone can help me.

Thanks,

Ramesh.