In accounting for inventory, original cost is critical. You can't decrease the original cost. You can write down the value of your inventory using the original cost by removing certain items for damage, shrinkage (theft), promotion (give away) etc.
The entry would be minus inventory and plus cost of goods or plus shrinkage or plus promo.
You might sell goods for less than the original cost to clear old stock, which reduces your gross profit, but what you paid for it remains the same.