He replied:

There is a well-known disagreement concerning this issue. If a man sells someone wheat, barley or another commodity subject to the rules of ribā on deferred payment, then accepts in place of its price wheat, barley or another commodity that may not be exchanged for the original commodity with deferred delivery, Mālik, Ahmad and others hold that this is impermissible. Abū Hanīfa and al-Shāfiʿī hold that it is permissible, as do some of Ahmad’s followers.

He was asked:

What about someone who sells pure silver for debased silver, exchanging one dirham for one and a half dirhams?

He replied:

Silver may only be exchanged for silver in equal quantities. If the alloying material in the silver is not itself sought in exchange for the silver, the transaction is permissible. However, if the amount of silver on one side exceeds that on the other, it is impermissible—particularly if the silver content of the debased silver exceeds the amount of pure silver given in exchange. This is impermissible by the consensus of the Muslims. Not knowing whether the quantities are equal is treated like knowing that they are unequal.