link REGUH and BKPF

Hi all friends, i search and search on the scn.sap.com and google.com but not find a answer satisfactory.

i have a Document Number(BELNR) and or Reference Key (AWKEY) of BKPF table

The process was:

1) Purchase Order

2) Purchase

3) Payment

anyone have a idea? how to link the tables?

Thanks.