Contract creation for Subcontract without Material

Dear All,

Client has a contract with the Vendor to maintain spare parts. The contract doesn't mention a material in specific, as in the Vendor had to process/ service any Material that the client gives them.

Currently they create a Service PO to handle this. But it will NOT track the Material, live, in SAP.

We proposed the use of Refurbishment Process to handle it. However, in that case, to call a contract, the contract (MK) asks for a Material as Mandatory. Secondly Item Category L is NOT allowed with WK.

How can we utilize Subcontracting Contract (with Material Unknown) (or) rather is it possible to create such a Contract and create Release/ Call Off Orders against it.

Regards,

Prasanna