Hello,
I am guessing this PO is for a Subcontractor... If so, go to the Posted Purchase Invoice, use the function Correct > Create Corrective Credit Memo.

This will create a Purchase Credit Memo to off-set the incorrect Sub-contracted Purchase Order. Your Production Order actual cost should Zero out the incorrect posting. When you run the Sub-Contracting Worksheet again it will suggest the Purchase Order again, and you would proceed as if the mistake hadn't happened.
Be aware, this will create a new PO, so the Vendor's invoice will likely reference the old PO prior to the mistake.
Hope this helps.
Thanks,
Steve