either their function as a medium of exchange or their quality as food, thereby indicating that usury arises only in these two categories: things that serve as a price or things that are consumed. This includes wheat, barley, dates, and even salt—though usually acquired for purposes other than eating—so that it is clear that all of them share the same ruling.
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These six items are the expressly mentioned ribā commodities; everything else has been assimilated to them by the scholars through qiyās (analogical reasoning).
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When one commodity is exchanged for another of its own genus—such as gold for gold or wheat for wheat—the validity of the contract depends on two conditions:
a. Parity: neither amount may exceed the other. This is what is meant by his words, “like for like,” and, “do not give more of one than the other.”
b. Immediate mutual possession in the session of contract. This is what is intended by his words, “hand to hand,” and, “do not sell what is absent against what is present.” -
If the exchange is between two different genera—such as gold for silver or wheat for dates—only one condition applies: that both parties take possession during the contract session. This is what is meant by his words, “hand to hand,” and, “do not sell what is absent for what is present.”
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A genus (jins) is a distinct name that covers several kinds, whereas a kind (nawʿ) includes individual items that differ in their particulars. At times a kind may function as a genus and vice-versa. Here, the intended genus is the specific one, like wheat, not the broader category of grain; and the intended kind is the specific variety, such as al-luqaymī, not the broader kind which is wheat itself.
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The scholars unanimously agree that any disparity is forbidden when one of these six categories is exchanged for its own genus, as stated in the ḥadīth of ʿUbādah ibn al-Ṣāmit.
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They also unanimously agree that disparity is permissible when two different genera are exchanged, provided that possession is taken in the same session, because of his statement: “When these kinds differ, sell as you wish, so long as it is hand to hand.”