—The Form of the Prophet’s Prohibitions—
Al-Shāfiʿī, may Allah have mercy on him, said:

532 – The default rule concerning any prohibition issued by the Messenger of Allah, peace and blessings be upon him, is that whatever he forbids is unlawful, unless evidence from him proves that he meant the prohibition for some reason other than declaring it unlawful—whether he intended it to apply to only certain cases and not to others, or he intended it merely as a discouragement for the sake of refinement, etiquette, and preferred conduct.

533 – We do not distinguish between one prophetic prohibition and another except on the basis of evidence from the Messenger of Allah himself, or on the basis of a matter about which the Muslims are unanimously agreed. For we know that all the Muslims together cannot be ignorant of a sunnah, even though some individuals might be.

535 – Among the things the Messenger of Allah, peace and blessings be upon him, forbade with the force of prohibition—about which most of the general public have never differed—is that he prohibited exchanging gold for silver except hand-to-hand, and gold for gold except equal for equal, hand-to-hand; and he forbade “two sales in one sale.”

535 – Accordingly, and the common people are with us in this, if two parties exchange gold for silver, or gold for gold, and each does not take possession before they separate, the sale is void.

536 – Our proof is that when the Prophet, peace and blessings be upon him, forbade it, it thereby became unlawful.

537 – Likewise, if two men conduct two sales within a single transaction, both sales are nullified as soon as they are contracted—for example, when I sell you something on the condition that you sell me something—because the contract is based on each party transferring ownership of something that is not yet in his possession.

538 – The Prophet, peace and blessings be upon him, also forbade sales involving gharar (excessive uncertainty). One instance is my saying, “This commodity of mine is yours for ten in cash or fifteen on credit,” for one of the two prices would become binding even though the sale has not been concluded for a known consideration. There are many forms of gharar-based sales; this example will suffice for now.