for each of these qualities carries an effect, and the legal ruling is associated with all of them in the revealed text (nass), so it is not permissible to omit any of them. Furthermore, measurement, weight, and genus do not in themselves necessitate the obligation of equality; rather, their effect is in establishing it. The 'illah (effective cause) is what requires the establishment of the ruling, not that whose condition has been realized. Mere taste does not suffice for equality to be realized, due to the absence of a Shari'a standard for it. Equality is only obligatory in the Shari'a standard, which is measurement and weight; therefore, it was necessary for taste to be considered in measured and weighed items, and not in others. The hadiths recorded in this chapter must be reconciled, and each must be qualified by the other. Thus, the prohibition of the Prophet (peace and blessings of Allah be upon him) from selling food except like for like is qualified by what has a Shari'a standard, which is measurement and weight. And his prohibition from selling a sa' for two sa's is qualified by the edible item in which an increase (tafadul) is forbidden.
Malik said: The 'illah is staple food (qut), or what serves as staple food from a single genus of stored items. Rabi'a said: Riba applies to items in which Zakat is due, and nothing else. Ibn Sirin said: A single genus is the 'illah. This opinion is invalid because of the statement of the Prophet (peace and blessings of Allah be upon him) regarding the sale of a horse for horses, and a riding camel for camels: "There is no harm in it if it is spot-for-spot (hand-to-hand)." (9) It was also narrated that the Prophet (peace and blessings of Allah be upon him) purchased one slave for two slaves. Recorded by Abu Dawud and al-Tirmidhi (10), who said: It is a hasan sahih hadith. The opinion of Malik is invalidated by firewood and seasonings used to improve food, for there is no Riba in them according to him. And the reasoning of Rabi'a is refuted by salt, and the reverse (al-'aks) is a necessary implication when the 'illah is unified.
(8) In the original: "tahqiquhuma" (the realization of both of them). (9) Cited earlier on page 54. (10) Recorded by Abu Dawud, in: Chapter concerning that when it is hand-to-hand, from the Book of Sales. Sunan Abi Dawud 2/225. And al-Tirmidhi, in: Chapter of what has been narrated regarding purchasing a slave for two slaves, from the Chapters of Sales. 'Aridat al-Ahwadhi 5/247. It was also recorded by Muslim, in: Chapter of the permissibility of selling animals for animals of the same genus with a difference in quantity, from the Book of Mu'amalat. Sahih Muslim 3/1225. And al-Nasa'i, in: Chapter of selling slaves, from the Book of Allegiance, and in: Chapter of selling animals for animals hand-to-hand with a difference in quantity, from the Book of Sales. Al-Mujtaba 7/135, 257. And Ibn Majah, in: Chapter of Allegiance, from the Book of Jihad. Sunan Ibn Majah 2/958.
