—that is, selling a ribā-liable commodity for its very source material, such as oil for olives. We shall return to this principle later, for it is what the jurists refer to as muzābanah. From one angle it falls under the heading of ribā and from another under the heading of gharar: when the items concerned belong to the six ribā categories it is forbidden on account of both usury and uncertainty, whereas in goods not subject to ribā it is prohibited only because of the gharar that arises from ignorance of what will be obtained beyond the principal.

A separate issue: within this same framework the jurists disagreed about selling flour for wheat on an equal-for-equal basis. The view most widely ascribed to Mālik—indeed, the one recorded in his Muwaṭṭaʾ—is that it is permissible. A contrary report from him, however, declares it impermissible; that is also the position of al-Shāfiʿī, Abū Ḥanīfah, and Ibn al-Mājishūn, a Mālikī authority. Some Mālikīs argue that this is not a true contradiction in Mālik’s statements: the narration that forbids the sale applies when equality is assessed by volume, because grinding wheat into flour changes its bulk, whereas the narration that permits it applies when equality is assessed by weight. Abū Ḥanīfah’s reason for prohibition is that one of the two commodities is ordinarily measured by volume and the other by weight. Mālik, by contrast, looks to volume or weight in whatever is customarily measured or weighed, and to number in whatever is usually counted rather than measured.

They also differed, again within this chapter, about processed items whose originals are ribā-susceptible, such as bread for bread. Abū Ḥanīfah considered such sales unobjectionable, whether equal or unequal, on the grounds that manufacturing removes the item from the genus in which ribā operates. Al-Shāfiʿī ruled them impermissible even when the two amounts are equal—let alone when they differ—because processing alters the substance so much that the very measures needed to establish equality become unknown. Mālik’s better-known position regarding bread is that it may be exchanged when the two portions are equal, although some report that both equality and disparity are allowed. As for dough for dough, he permits it provided the amounts are equal.

The crux of the disagreement is whether processing transfers the item out of the ribā genus. If it does not, can genuine equivalence still be ascertained? Abū Ḥanīfah says that processing does transfer it, whereas Mālik and al-Shāfiʿī say that it does not. They then part company over the possibility of establishing equality after processing. Mālik will accept equality in bread and meat through estimation and visual appraisal as well as through weighing. If only one of the two ribā items has been processed, Mālik often holds that processing removes it from the common genus and therefore allows disparity, though in some cases he does not. Detailing his school’s nuances here is difficult: roasted and boiled meat count for him as one genus, whereas roasted wheat and unroasted wheat are two genera. His disciples have tried to codify these distinctions, but the apparent sense of his teaching is that no single rule gathers all his remarks. Al-Bājī attempted such a codification in al-Muntaqā. By the same token, it is hard to catalogue the kinds of benefit that, in his view, make two items identical for transactional purposes—and to distinguish them from those that do not—whether in animals, movables, or produce. The difficulty arises because, when a person is asked about items that closely resemble one another in