automatic PO form goods receipt without reference Movement type 501

Hi,

My client receive sample material from the vendor that is without reference to PO, but upon approval of sample the vendor has to make the payment for which i do not have the reference document number.

To arive at the solution i wanted to generate automatic PO for movement type 501, for which i have checked the movement type settings, 501- automatic PO, i ahve maintained the info record, and during GR for movement type 501 i enter the vendor even than system is not generating any PO in backend, I have centraalised purchase organization assigned to the plant.

can anyone suggest me if i am missing any settings,

Suggestions willbe worth appreciating..\

Thanks

Sarf