He said: “Whoever pays in advance for something, let him do so for a specified measure, a specified weight, and a specified term.” (1)
The point of evidence in this hadith is that a salam sale requires the price to be paid in advance and the goods to be deferred. This is a deferred transaction, and the Prophet, peace and blessings be upon him, approved of it. It is known that advance payment is made in dirhams, which are weighed, or in dinars, which are weighed. This is why he said, “or a specified weight.” This shows that, if one of the two exchanged items is currency, deferment is valid; otherwise, the avenue of salam transactions involving weighed goods would be closed. For if we did not recognize this exception, salam in weighed goods would not be valid when the price was paid in dirhams or dinars.
It is known that when one of the two items is currency and a commodity measured by volume is purchased with it, deferment is permissible, because the two differ with respect to the effective cause of excess usury. But the difficulty addressed by this exception arises when payment in advance is made for an item sold by weight. Were it not for this exception, we would say that salam in weighed goods is impermissible unless the advance payment is made with something other than gold or silver.
The reasoning is that, had the Prophet not permitted deferment in the sale of currency for weighed goods, the avenue of salam transactions in weighed goods would generally have been closed. By our saying “generally,” we mean that one might say that salam in weighed goods would not be closed, since the price could be made a commodity measured by volume. For example, one could say: “I have paid you in advance one hundred sa‘ of wheat for a ton of iron.” In this case, deferment is permissible because the effective causes differ: the two items do not share the effective cause of excess usury. Therefore, we say that the avenue of salam transactions in weighed goods would generally be closed, not always, because salam may be conducted with something other than a weighed commodity, such as goods measured by volume or animals.