Dear cha wa
I think Jurgen is right. ... I want to explain one of the scenario which i have faced similar to yours. ..We had two company codes 1000 and 2000 where 2000 company code is an umbrella company which takes cares of the procurement of company code 1000 and has a service charge for this activity. Sales happens from company code 1000 to customers.
For this process we create a standard PO from company code 2000 to the vendor and do GR and create a STO from company code 1000 and replenishment inVL10B and outbound delivery from 2000 and a GR for the plant in company code 1000. vendor invoice and payment is processed for company code 2000.
In this case even we can create a sales order in company code 1000 and link it to the STO with the Account assignment category.
Regards
Irfan Ahmed