Does Zakat on money become obligatory on an amount that has decreased by more than half, given that a quarter of it was loaned to two people, and with the remainder a car and a shop were purchased, and the remaining portion with the brother is diminishing?

If the remaining money reaches the nisab (threshold) and a full year has passed over it, then zakat is obligatory on it. As for a debt that was lent, if the debtor is in straitened circumstances or procrastinates, then zakat is not obligatory on it until it is collected, and then zakat is paid for one year. However, if the debtor is solvent and does not procrastinate, the debt is considered like a deposit, and zakat is paid on it every year. A car, if it is for personal use, has no zakat on it. But if it is for rent, then zakat is due on its earnings. A commercial shop is subject to zakat on its goods prepared for sale at the end of the year, but the building itself and its machinery have no zakat on them. If the shop is rented out, then zakat is on its earnings.