Is the riba of sales limited to the six commodities?

The jurists agree that riba is forbidden and that it occurs in the sale of the six commodities named in the hadith: gold, silver, wheat, barley, dates and salt. They differ over other goods: were the six named to point to a cause that extends to whatever shares it, or are they the only place of sales riba? Ibn Hazm sets out the question in the Book of Sales of al-Muhalla and argues for the second view. ^{1 2}

Ibn Hazm lays down that riba is forbidden in sales and forward sales only in six things, dates, wheat, barley, salt, gold and silver, while in loans it occurs in everything, so that nothing may be lent to be returned less or more or in another kind, but only the like of what was lent in kind and amount; this, he says, is certain consensus, the difference being that sales and forward sales can be of a kind for its own kind or for another, while a loan is only of a kind for its own kind. ¹ He says that riba in the six kinds in sales and forward sales is a matter of consensus, while everything else is disputed. ¹ He cites, on the gravity of riba, al-Baqara 275 and the verses after it, the hadith "avoid the seven destroyers", which counts consuming riba among them, and Ibn Mas'ud's report that the Prophet ﷺ cursed the one who consumes riba and the one who pays it. ^{1 5}

Ibn Hazm builds on al-An'am 119, which says God has set out in detail what He forbade: whatever riba or other prohibition is set out through His Messenger is riba and forbidden, and whatever is not set out is lawful; were there something in the law that God forbade without setting it out, and His Messenger did not explain it, that would contradict the verse. ^{5 2} He names, among those who held that there is riba only in the listed kinds, Tawus, Qatada, 'Uthman al-Batti, Abu Sulayman Dawud and all his school. ²

Ibn Hazm reports that one group took the six as pointing to whatever resembles them in a cause, so that riba occurs wherever that cause is found, and that they then differed on the cause, each group refuting the others'. ² Some made it taste and colour, like Ibn Shihab al-Zuhri on chickpeas for lentils, and the same is reported from Ibn Mas'ud, Yahya ibn Sa'id and Rabi'a; Rabi'a also made it liability to zakat, which Ibn Hazm answers by noting that salt carries no zakat yet riba occurs in it by text. ^{2 3} He reports that al-Shafi'i made the cause being food together with being a measure of price, and Malik being stored food together with being a measure of price, and regards both, like the view of Rabi'a's followers, as claims without proof. ³ He mentions other views: al-Hakam ibn 'Utayba saw no harm in fifteen jaribs of land for ten while Hammad ibn Abi Sulayman disliked it; Sa'id ibn Jubayr made the cause closeness of use within a kind; and Muhammad ibn Sirin allowed one for two hand to hand when the kinds differ. ^{3 6}

He reports that Abu Hanifa and his followers made the cause being measured by volume or weight within one kind or two: a measured good is sold for its own kind measure for measure, hand to hand, with neither excess nor credit, and for another measured kind with excess hand to hand but not on credit; weighed goods likewise, except that gold and silver may be paid on credit for other weighed goods; and a measured good may be sold for a weighed one with excess and on credit, as meat for wheat or honey for dates. He notes that some of their later scholars moved to "volume or weight in what is specified" because the cause broke down for them over gold and silver. ⁴

The schools' views are given only as Ibn Hazm reports them; they have not yet been added from the schools' own books, nor the Hanbali view, nor what Ibn Hazm says later about the hadiths on excess and credit and the view of Ibn 'Abbas.