What are the rulings of Waqf in Islamic Sharia?

There are three types of waqf (endowment): al-mu'aqqab (intergenerational), al-'umari (lifelong), and al-ruqba (mutual reversionary - which is impermissible). A condition for a waqf is that the endowed asset must be usable. It is not permissible to endow gold, silver, food, or drink. It is permissible to make a waqf for Dhimmis (non-Muslims living under Islamic rule) and for the wealthy. It is valid to endow an undivided share. It is not valid to endow for oneself because one of the pillars of a waqf is the beneficiary (al-mawquf lahu). If the waqf is made during a death-illness for some heirs, it is not valid unless they all give their consent. It is permissible for a worker to consume from the waqf if it is considered wages. Any surplus profit from the waqf should be spent on similar charitable purposes.