How is the Zakat calculated for a person who buys and sells used clothes in separate installments, pays their costs from his own money, spends his profits immediately upon receiving them, with a portion of the clothes remaining unsold, and he currently possesses the price of the last installment but has not yet spent it?

Zakat is not obligatory on trade goods unless they reach a nisab (minimum taxable amount) when a complete lunar year (hawl) passes over them, or when a hawl passes over the capital with which they were purchased, provided that capital reached the nisab (85 grams of pure gold or 595 grams of silver).

So, if a complete lunar year has not passed over this man's wealth, which reached the nisab, then there is no zakat due on him. However, if a complete year has passed over the capital with which he purchased them while they are in his possession, and this capital reached the nisab, and he intended them for trade, then zakat becomes obligatory on him. He should then appraise these trade goods on the day the year completes and pay a quarter of a tenth (2.5%) of their value.

He should do this for all the years he delayed paying their zakat, and he must seek Allah Almighty's forgiveness for his negligence in fulfilling the zakat obligation on time.