What is the amount of zakat due on the sale price of a plot of land, and is the same amount subject to zakat every year, or only for the current year?

Zakat is obligatory on permissible investment certificates, both on the principal and its profit. Its amount is a quarter of a tenth (2.5%) when a year has passed on the original capital. As for prohibited investment certificates, one must repent from them, and Zakat is paid only on the principal, not on the profit. The profits must be disposed of by spending them on charitable causes.