I would suggest starting with a non-order connected RMA using a made up inventory part number called ‘CUSTOMER OWNED’ or something that readily identifies it as such.  Process and receive the RMA with no credit of course and put the item in inventory.  Set up a warehouse for customer returns and create a bay/row/tier unique to the customer ID to isolate the customer material within the system.  Then when you setup the shop order, you can tie that customer item with the specified inventory part directly to that location.  Doing this ensures that the shop order will look in this location first for available material if for some reason someone backflushes the shop order first rather than manual issuing.

Lots of details to work out that go into design of the process, but that sketch I’m sure will work for most scenarios.

We are on Apps10 UPD6 but we had similar requirements in Apps8 SP1+ for repairs.  We used POs where the customer is setup as the supplier (you can associate them in Supplier setup) and setup the PO as “customer owned” - meaning you aren’t paying them and it can only be used for that customer (not available for sale).

If you are bringing in the material to have it on hand .  You can try that - see if you can use that material on a shop order pegged to a customer order for that same customer.

If you are doing a more “as needed” purchase model, you can try manually pegging the PO to the shop order material line.