How is the zakat calculated on money available from a car rental agency if it is continuously allocated to purchase new cars to strengthen the business and does not reach the minimum threshold (nisab)?

Zakat becomes obligatory on rental income if it reaches the nisab (85 grams of gold or 595 grams of silver) and one lunar year (hawl) passes over it, whether by itself or by adding it to other money, gold, or silver. There is no zakat if it does not reach the nisab, or if it reaches the nisab but a year has not passed over it because it was spent on necessities or buying cars. Similarly, there is no zakat on the value of cars unless they are intended for trade.