Contract without Vendor ?

My Requirement is to record, control and approve a quarterly RAW material Spend in the SAP purchasing area. The RAW material quarterly forecast spend is not assigned to a vendor(s). Periodically (weekly), Purchase orders are to be placed with suppliers and, in total, are not to exceed the value or date range of the approved forecast.

So this looks like a good fit for value based contract, except Contracts require a Vendor assignment. and the forecast is not for a specific Vendor.

So this looks like it has to be a Requisition, because vendor is not required field, but how to restrict PO processing with reference to this PR for Date and Value, while still Issuing a material master, inventoried item ?

Any suggestions are greatly appreciated !

Brock