That is, I bring a sāʿ, a quarter-sāʿ, or a small cup of wheat and say: “I sell you the equivalent of this sāʿ for such-and-such.” This establishes the description through seeing a sample. I have not seen all of it, but I have seen the cup, and he says: “I have food like what is in this cup.”

The answer is that the scholars differ over this. Some hold that it is invalid,(1) but the correct view is that the sale is valid, because knowledge is attained in this manner, and people have continued to transact in this way.

His statement, “If he buys something he has not seen”: an example is where he says, “I sell you such-and-such car for such-and-such,” without the buyer having seen it; or, “I sell you a book for such-and-such,” when he has not seen it. The sale is invalid. However, if he describes it, the sale is valid, provided it is something that can be precisely defined by description. This is why the author—may Allah have mercy on him—said in al-Rawḍ (2): “Something he has not seen, without a description.” It may be said that the author of the foundational text was deficient here; or it may be said that he was not deficient, on the basis that knowledge may be attained through seeing and through description.

His statement, “Or he sees it but does not know what it is”: that is, he says, “I sell you what is in this sack,” while he does not know whether it is sand or sugar. The sale is invalid because he does not know.

But if he is unaware of its use while knowing what the item itself is—for example, he sells him a mechanical device, but he does not know what it does—should this be deemed sufficient knowledge, such that the sale is valid, or should it be deemed insufficient knowledge, such that the sale is invalid?