Write Off functionality in Inventory module

Hi

We need to write off obsolete/slow moving items from Inventory, but the item will still remain in Inventory but the cost of the item must be ZERO.(we have some 8,000 items)

Also note that we use Periodic Costing(PAC)

For this
1-We processed Misc. Receipts from Inventory (unit cost is left blank for the system to take the cost automatically)
2-We processed Misc receipt with cost equal to zero.
In Inventory module the cost became ZERO as expected (average costing), however in Cost Manager the cost is NOT EQUAL TO ZERO, it is half of what it was before we processed Misc. Issue+misc. Receipt.