Subcontract an Item with a delayed PO

I have a scenario
There are few items that i subcontract (Raw material Supplied)
Raw material is supplied by us to subcontract
Required Item and BOM is already created
The Problem Is
I cannot raise an PO because the price shall be discussed only after the manufacturing is over
so what i do is
I supply the material to Subcontract thru Stock Entry
Eg… Item A — Qty Sent —10nos
I receive the material thru Purchase Reciept
here i have the provisions to select the item recieving and add prices and tax
the stock entry and Purchase Reciept is not Linked
even though the system knows that item A 10nos is been sent to subcontract (stock entry selection) for process and becomes Item B
when i recieve the material thru Purchase Reciept, it should be able to accept the converted process item noB (fetched from the relative BOM)
the quantity supplied by the subcontract also should be validated

Stock Entry and Purchase Receipt is not linked anyways.

When you receive Purchase Receipt, you have to select sub-contractor’s warehouse. When Purchase Receipt is submitted, the stock of raw-material item should be present in the subcontractor warehouse, so that it can be consumed, hence transfer entry is created before receipt entry.

Even if rate is not confirmed, you can enter estimated rate in the Purchase Order, but amend the rate in the Purchase Receipt and Purchase Invoice.