As for the prohibition of deferred payment (nasīʾah) in these commodities, it is established by numerous ḥadīths; the best-known is the report of ʿUmar ibn al-Khaṭṭāb, who said:
“The Messenger of Allah—peace and blessings be upon him—said: ‘Gold for gold is riba unless it is hand to hand; wheat for wheat is riba unless it is hand to hand; dates for dates is riba unless it is hand to hand; and barley for barley is riba unless it is hand to hand.’”

The ḥadīth of ʿUbādah therefore entails the prohibition of excess (tafāḍul) within the same genus. It also, in some sound narrations, entails the prohibition of deferment between two of these genera while permitting excess, for they add after the ban on excess among the six items: “Sell gold for silver as you wish, provided it is hand to hand; and sell wheat for barley as you wish, provided it is hand to hand.”

All of this is unanimously accepted by the jurists—except for the case of wheat against barley. Concerning commodities other than the six expressly mentioned, the scholars disagreed. A group of them, the Ẓāhirīs, held that excess is forbidden only within each of these six kinds and that, in everything else, excess within a single genus is allowed. The same scholars further maintained that deferment is barred only in these six items, whether or not the two things exchanged are of the same genus; this consensus—namely, that deferment is prohibited among the six even when the genera differ—admits one reported exception: Ibn ʿUlayyah, who said that when the two genera differ both excess and deferment are permissible, apart from gold and silver. Accordingly, these scholars treated the prophetic prohibition tied to the six items as a specific ruling intended to remain specific.

The majority of jurists in the great centers, however, agreed that the ruling is a specific instance intended to indicate a general principle, though they diverged over what that general principle is—that is, over the operative cause for forbidding excess and deferment in these items.

The conclusion adopted by the leading Mālikīs is that the reason excess is forbidden, with respect to the four foodstuffs, is their being a single genus of provisions that are both storable and commonly eaten as staples. Others within the school said the criterion is simply that they form a storable genus, even if not normally eaten as staples, provided that, for the most part, they are indeed stored. Yet another view within the school is that riba applies to anything storable, even if it is seldom stored.

In their eyes, the cause for prohibiting excess in gold and silver is likewise the fact that they constitute a single genus, together with their role as the standard of prices and the measure of indemnities. They call this rationale al-qāṣirah (“restricted”), because, in their judgment, it is found only in gold and silver.

As for the Mālikī rationale for forbidding deferment in the four named items, it is their being edible and storable, without requiring sameness of genus. Hence, when the genera differ they permit excess but not deferment; likewise they allow excess within a single genus of foodstuffs that are not storable, while deferment remains impermissible. The permission of excess rests on the fact that such items are not storable,