Halo Juergen,
In this business, the purchasing document (scheduling agreement or PO) is created from a different legacy system and flows to SAP. Purchasing activity is taken care by a different system.
Staging area is in the same location. Any other solution, could there be any chance like the below illustration
SA# 5500000000
DD Qty Stagng area #
15.01.2015 15 X
15.01.2015 10 Y
The same info requires to be triggered via SA release to vendor and accordingly communication is carried out.