Shaykh al-Islām—may Allah have mercy on him—said:

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Contracts involving some form of exchange—which constitute most human transactions and are indispensable to human life—fall into four categories. Such exchanges may involve property for property, as in sales; a benefit for property, as in leasing and contracts for a stipulated reward; or a benefit for another benefit, as in mutual assistance, mutual support, and similar arrangements. Governorship, military recruitment, and other such public appointments may also fall within this discussion.

They may be permissible for both parties, as in sales, leasing, and cooperation in righteousness and piety.

Alternatively, they may be forbidden for both parties, as in exchanging wine for swine, hiring someone for illicit sexual intercourse in return for wine, or hiring someone to give false testimony in return for false testimony. As one judge used to say of a group of leaders: “They lend one another false testimony”—likening this to an exchange of loans.

Or they may be permissible for one party and forbidden for the other. Muslims ought to understand this category, for neither religion nor worldly life can be sustained without it.