Be careful because if you perform a negative work order issue, the parts may not come out of the work order at the SAME cost that they went into the work order on the PO Job Receipt.
 
We usually re-open the PO, ask purchasing to modify the PO so that the unused pcs wil go to stock, rather than a work order.
We enter a negative receipt qty to remove these pcs directly from the original work order, then re-receive the unused items on the new PO line to make them go to stock.
 
Remember to inform Accounts Payable, that these two transactions must be cleared by them via AP-C, and then they can re-close the purchase order.

If it doesn't matter that the cost of these unused parts is in the work order job costs,
the EASIEST way to get these parts back into inventory is to use IN-C, and add them back to inventory at $0.  (there is no scrap process required)
 
This keeps inventory count accurate and the next time you need to use these parts, the work order's profitabily will increase from the $0 value of the parts.