taking possession in the sale of every two commodities that share the same effective cause for excess usury must occur before the parties leave the contractual session. This is because a sale involves either a single usurious category, two usurious categories that share the same effective cause for excess usury, two usurious categories that do not share the same effective cause, or two things that are not subject to usury. The cases are therefore four:
First: If the sale concerns a single usurious category, both disparity in amount and deferment are prohibited.
Second: If it concerns two usurious categories that share the same effective cause for excess usury, only deferment is prohibited between them, not disparity in amount.
Third: If it is between two usurious categories that do not share the same effective cause, both disparity and deferment are permissible.
Fourth: If it is between two things that are not subject to usury, everything is permissible: disparity in amount and deferment.
His statement, “in the sale of every two commodities that share the same effective cause for excess usury”: “every two commodities” excludes a single category, for a single category is subject to both deferred-exchange usury and excess usury. The author is now explaining deferred-exchange usury. Thus, we state the following principle: “Whatever two things are subject to excess usury between them are also subject to deferred-exchange usury; but not the reverse.” This is why he said, “in the sale of every two commodities that share the same effective cause for excess usury.” What, then, is the effective cause for excess usury?
The answer is that we previously mentioned, at the beginning of the chapter on usury, that the scholars differed over it. The position of our school—the well-known view followed by the later scholars among the companions of Imam Ahmad—is that the effective cause is measurement by volume and weight, whether the item measured by volume is edible or not, or whether it is a staple food or not; and likewise