weighed commodities. Thus, whenever two commodities share measurement by volume, deferred-exchange usury applies between them, and they are unquestionably both subject to usury, since the effective cause is measurement by volume.
His statement, “share the effective cause for excess usury”: if we sell one commodity measured by volume for another commodity measured by volume of a different kind, taking possession before the parties leave the contractual session is obligatory, whether the commodities are edible or inedible. For example, if we sell a sa‘ of wheat for a sa‘ of ushnan—ushnan is a powder resembling the soap used for washing clothes; it comes from a tree that is dried and ground, and it is sold by volume—then, if one sells a sa‘ of barley for a sa‘ of ushnan, taking possession before the parties leave the contractual session is obligatory, but equality in amount is not obligatory because the kinds differ. This is due to the Prophet’s statement: “When these categories differ, sell as you wish, provided it is hand to hand” (1).
His statement, “as you wish,” means with an increase or a decrease, but it must be “hand to hand.” Likewise, if one sells a sa‘ of wheat for a sa‘ of gypsum, taking possession is essential, because gypsum is sold by volume. In former times, they used to sell it by volume: one would come to the seller and say, “Give me a sa‘ of gypsum,” and he would give it to him.
His statement, “Neither of the two is currency”: if one of them is currency, deferred exchange is not prohibited, just as disparity in amount is not prohibited.
An example is selling iron for dinars. The effective cause for excess usury is present in both, since they are both weighed. Accordingly, the principle would require deferred exchange to be prohibited. However, the author, may Allah have mercy on him, made an exception and said, “Neither of the two is currency.” The evidence for this exception is the hadith of Abdullah ibn Abbas, may Allah be pleased with him, that the Prophet ﷺ came to Medina while its people were paying in advance for fruits for one year or two.