Can we Create & Use the Approved Supplier List for "Fixed Price Services" Line Type

Summary:

We have a business requirement to Procure the Services from the Vendors by raising a Purchase Requisition and there by creating a Purchase Order.

But the business users would like to Restrict the procurement of Services only to Specific Vendors and the business users do not want to Create any Item Code or use any Item Master.

We created an ASL with a specific Supplier with the "Item Category" (not an Item) and we have associated a Contract Purchase Agreement to this ASL.

We then tried to create a PO with the Line Type as "Fixed Price Service", selected the same "Category" which was used while creating ASL and selected a different supplier which is NOT Used in ASL.