We have 2 FIFO costed items that were incorrectly received and billed at the wrong costs (swapped). The difference is rather significant. The periods are already closed and cannot be re-opened. What is the best way to fix it? If I do an inventory adjustment to do a negative then a positive, the cost related to negative quantity would be the earliest available FIFO cost (and not the actual cost I am trying to fix).

Inventory Worksheet is the cleanest way if you can't open the period. But that would only fix the on hand. Have you used any since they have been received?

Inventory worksheet resets all the FIFO cost layers though. Yes, we've already recording sales related to those.