Correcting material costs on a Completed work order

We discovered that an item in our inventory was putaway at an incorrect cost..much higher than purchased. The costs have carried over to work orders that were issued this item, some of which have already been "completed" so we cannot return the items to inventory, and re-issue at the correct price. How should we correct the costs of the "completed" work orders so as to unencumber the funds that the erroneous price has created?