Reconcile Inventory Qty and cost for wrongly entered PO receipts which has already been sold

There is a By-product item called BYP-A,which has a cost of 100 (standard).There are wrongly raised POs (PO Qty-very high compared to actual qty and PO unit price-3;very low compared to actual price ) which has been GRN and sold to customers through sales issue.After running actual cost processes, Now the average item cost for the BYP-A is 23.Inventory valuation report Qty for that item has been very high.How to correct the cost and the inventory? (I tried Actual cost adjustment to correct cost.but I don't know how to correct inventory?)

Regards,

Ayeshan