for barley, dates for dates, and salt for salt—equal for equal, hand to hand.’ It was narrated by Imam Aḥmad and Muslim. The ḥadith, therefore, forbids selling gold for gold unless it is equal in amount, exchanged on the spot, with no excess whatsoever; the same applies to wheat for wheat, barley for barley, dates for dates—of every variety—and salt for salt: they must be identical, equal for equal, hand to hand.

By analogy, everything that shares the same effective cause with these six items falls under the same ruling, so any surplus is prohibited according to the majority of scholars, although they differ over what that cause is.

The sound position is that the cause in the two monetary metals is their function as a medium of exchange; therefore, anything adopted as a price—such as the paper currency in use today—follows the same rule. It is forbidden to sell some of it for more of the same kind when the “species” is one—that is, when the notes are issued by the same state.

The correct view concerning the remaining four categories—wheat, barley, dates, and salt—is that the cause is that they are edible items sold by measure or by weight. Consequently, every foodstuff that is measured or weighed and shares this characteristic is likewise subject to the prohibition of riba al-faḍl.

Shaykh al-Islām Ibn Taymiyyah (may Allah have mercy on him) said: “The effective cause in the prohibition of riba al-faḍl is when a commodity is edible and sold by measure or by weight; this is the position related from Aḥmad.” End of quote.