There are a couple of ways to handle this:
1) Use the adjust unit cost utility to update the costs. This won't change anything on the matched transactions to the PO (the shipment receipt and the invoice match), but will update inventory receipt costs and then create GL transactions to post to the GL with the cost changes.
2) Use Returns Transaction Entry to back out the shipment receipt and invoice, reopen the PO, and then enter/post the transactions again.
** The only way this works is if the inventory items a) have not been sold or b) if they were sold, they were put back into inventory with a SOP Return or Inventory Adjustment.
This is in Purchasing > Transactions > Returns Transaction Entry. Select either Return w/Credit (if the items haven't been sold yet) or Inventory w/Credit (if the items were sold and then put back into inventory). Enter a document number, batch, document date and vendor. Check the box "Replace Returned Goods."
If the items are unsold:
Look up the PO number, look up the associated receipt number, verify quantity and unit cost. Repeat this for every line you need to back out.
If the items were sold:
Enter the item number and quantity to return. Enter/verify the unit cost. Repeat this for every line you need to back out.
Check the distributions. Ensure the debit is to your accounts payable and credit is to inventory.
Post this batch.
Now, one of two things is going to happen. If the items were unsold, this will reopen the original PO and lines you just entered the return for. Then you can change the unit costs on the original PO, enter/post the shipment receipt, enter/post the match again.
If the items were already sold and were put back into inventory, this will create a new PO. Then all you have to do is update the PO with your costs, release it, enter/post the shipment receipt, enter/post the invoice match.
Lastly, apply the credit the GP created for the vendor when you posted the return transaction to the original invoice from the vendor.
I hope this makes sense and helps. Please reach back out if something is unclear or you hit a roadblock.
Thank you.
Joe