If someone gives some property to particular persons – for example, he gives his children some property as a charitable endowment (waqf ) – in the event that the property is farmed or trees are planted on it, and something is earned from it, and the earnings exceed their living expenses for the year, then those persons must pay khums on the extra earnings.
Similarly, if they profit from the property in some other way – for example, they give it on hire (ijārah) – they must pay khums on the amount that exceeds their living expenses for the year.