This is extremely simple if you are keeping an inventory:
The discarded inventory drives your ending inventory down which, in turn, increases your cost of goods sold. The higher your cost of goods sold, the lower your gross profit.
Of course, you need to do a year-end, physical inventory, but you should be doing that if you maintain an inventory. In the event your inventory is computerized and you do not take a physical count, you can simply reduce your ending inventory by the cost of the inventory you discarded.
