Hi,
Without knowing anything really about how you have setup inventory in your system, you could consider the use of the 'revaluation journal' functionality to correct specific Item Ledger Entries.
On the 'revaluation journal' you will set the 'Applies-to Entry' field to be the posted item adjustment entry that you want to change.
Keep in mind that the 'Unit Cost (Revalued)' value is the correct cost per unit and not the difference between the posted 'adjustment' value and the value that you want the cost to be.