for something, and the delivery date arrives but you cannot find what you paid for in advance, then accept a substitute of lesser value, and do not profit twice.” Likewise, Mālik’s school permits accepting a substitute at the price prevailing that day, as Ahmad ruled and related from Ibn ʿAbbās. Mālik excluded foodstuffs, because one of his principles is that foodstuffs may not be sold before taking possession of them. This is also one position reported from Ahmad. Ahmad distinguished between exchanging goods sold by measure or weight for other goods sold by measure or weight, and exchanging them for something else.

If they are exchanged for something else—for example, if someone accepts horses or cattle in place of goods sold by measure or weight, such as the wheat or barley for which he paid in advance—Ahmad permitted this, as did Mālik and, before them, Ibn ʿAbbās, provided the exchange is at the price prevailing that day. But if he accepts other goods sold by measure or weight in their place, such as barley in place of wheat, Ahmad disapproved of this unless it was for an equivalent amount. This is because exchanging goods measured by volume for other such goods, or goods measured by weight for other such goods, requires immediate settlement and both parties taking possession. Hence, wheat may only be exchanged for barley hand to hand, and gold may only be exchanged for silver hand to hand.

The advance-paying buyer has not taken possession of what is owed under the salam contract, so Ahmad disapproved of this, just as he, according to one of the two positions reported from him, and al-Shāfiʿī, according to one of his two opinions, disapprove of selling a debt to the person who owes it in all cases, on the grounds that this constitutes selling something whose risk one has not borne and of which one has not taken possession. The correct view, held by the majority of scholars and reflected in the apparent position of al-Shāfiʿī