HI Merten,
Good Morning.
Considering the requirement, I think standard SAP functionality of external service operation with PR/PO process is the solution but which is not desired in your case.
As a workaround, I could think of following option. You need to try and test it with MM person around you.
1) Try creating separate material type & material coding with unique identity of supplier & have desired price. During initial stock upload charge it to dummy GL..
2) Once you use this material in WO, with the help of MM, do the respective setting in OBYC to charge goods issuance to order & FI posting to the respective GL related to supplier.
This thought process needs to be thoroughly tested & checked
Regards,
Amol