I hope you have to pay the full material cost to the vendor for material B.
And you have to pay only service charges to vendor for material A.
Now you can see the difference..
The main process is that
You have already sent material A to the vendor..
And for material B, you should create a standard PO for material B, and then you have to do goods receipt with delivery address the same vendor..
If the vendor will take the payment for only subcontracting, then you can do a process.
Either use movement type 202 and give the stock from cost center..
Or use movement type 561 O..
(Note : 561 O is may be harmful for your accounting entry, before doing this please consult with your FI team)