How is the zakat calculated for a store selling old iron, some of which is not sold or whose price decreases?

The value of shop goods and cash liquidity is calculated at the end of each Hijri year, and the goods are appraised at their current price at the time the zakat becomes due. Then, zakat on the total amount is disbursed if it reaches the nisab (a quarter of a tenth). Goods whose price has decreased are appraised at their current market price. As for unsalable goods, there is a detailed explanation.