If he sells one sāʿ of dates for one sāʿ of wheat, deferment is not permissible, because they share the effective cause of excess usury, namely measurement by volume.

If he buys one sāʿ of wheat for one dirham, deferment is permissible, because they do not share the effective cause of excess usury.

If he buys one ton of lead for one hundred dirhams, deferment is permissible, because they share the effective cause of excess usury, namely weight; but an exception is made when one of them is currency.

The exception made for currency indicates that weighted goods are not subject to usury, as we previously held to be the stronger view. We said that the sound view concerning gold and silver is that the effective cause is their very substance and their monetary function. Thus, usury applies to gold and silver themselves, whatever form they take, as indicated by the hadith concerning the necklace (1). Moreover, they are people’s currency, the prices and measures of value for things—not because they are sold by weight. The validity of salam transactions also indicates this view, for the Messenger of Allah, peace and blessings be upon him, said: “or a known weight.” It is known that weighted goods will be bought with dirhams; this shows that weighted goods are not subject to usury. This is the correct view, except in the case of staple foods, which return to the other effective cause.

His statement, “If they part before taking possession, it is invalid”: thus, taking possession is a condition for the continued validity of the contract. That is, the contract has been concluded and is valid, but taking possession is required for its validity to continue. Hence he said, “If they part before taking possession, it is invalid.” The evidence is the hadith we mentioned.

For example, he sells him wheat for barley in the shop, but the barley is in