What is the obligatory Zakat on a sum of money saved in the form of gold, which was later sold to purchase agricultural land for investment purposes? And is the Zakat calculated from the time the Nisab is reached, or from the time the land is purchased, or from the time it is received?

Zakat is calculated according to the Hijri calendar. If you buy land with the intention of trade, its zakat is one-quarter of one-tenth of its value when one year has passed since its original purchase, which is gold. However, if you buy land without the intention of trade, there is no zakat on it. Zakat is only obligatory on its rental income if one year has passed since its receipt. For the validity of a rental agreement, the rent must be known. Some scholars consider a rental agreement valid with a customary rent (ujrat al-mithl) if the rent is not specified, such as entering a public bath or renting when the price is not named. Zakat on rent becomes obligatory when ownership is established by the tenant's utilization of the benefit.