Moreover, the Prophet (peace and blessings be upon him) prohibited certain things whose harmfulness may not be apparent because they lead to certain harm, just as he prohibited even a small amount of wine because it leads to consuming a large amount. One example is usury through unequal exchange, the rationale for whose prohibition may not be readily apparent. No rational person would exchange one dirham for two unless they differed in quality—for example, if the single dirham were intact while the other two were broken, if the single dirham had been fashioned into an article, or if it were readily accepted currency, and so forth. For this reason, the rationale for its prohibition was not apparent to Ibn Abbās, Muāwiya, and others, and they saw nothing wrong with it until senior Companions—such as Ubāda ibn al-Sāmit, Abū Saīd, and others—informed them that the Prophet (peace and blessings be upon him) had prohibited usury through unequal exchange.

As for prohibited uncertainty in sales, it is of three kinds. The first concerns something nonexistent, such as the sale of the offspring of an unborn animal or the sale of a tree’s future produce for a number of years. The second concerns something that cannot be delivered, such as a runaway slave. The third concerns something wholly unspecified, or a specified item whose category or quantity is unknown, as when someone says, “I sell you a slave,” “I sell you whatever is in my house,” or “I sell you my slaves.”

As for a specified item whose general kind and quantity are known but whose particular type or quality is unknown—as when someone says, “I sell you the garment in my sleeve,” “I sell you the slave whom I own,” or something similar—there is a well-known disagreement concerning it. [This is known as] the issue of selling specified items that are absent from view. Three positions are reported from Ahmad concerning it.

The first is that such a sale is never valid. This accords with al-Shāfiī’s later position.

The second is that it is valid even if the item was not described, and the purchaser has the option to cancel upon seeing it. This is Abū Hanīfa’s position. It is also reported from Ahmad that the purchaser has no such option.

The third—and this is the well-known position—is that the sale is invalid when the item is described and is also invalid when it is not described, just like a generic item owed as an obligation. This is Mālik’s position.