: “Whoever makes a loan must not stipulate anything better in return, even a handful of fodder, for that is usury.”
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This is explained by analogy with the usury forbidden in the Quran: the usury of the pre-Islamic period, expressed as, “Either repay or increase the debt.” Granting a debtor an extension after his debt falls due, on condition that he increase it, is a loan yielding a benefit. In a loan, accepting something better than what was lent is permissible only when this was not stipulated, as “the Messenger of Allah, peace and blessings be upon him, did when he borrowed a young camel from a man and repaid him with a choice, mature camel, saying, ‘The best of people are those who repay their debts best.’” Mālik—may Allah Most High have mercy on him—disliked accepting a greater quantity when a loan is repaid at the repayment session. There is no harm in accepting it after that session, provided there was neither a promise nor an established practice.
If someone advances goods or livestock in return for a greater number of the same category, or for something of superior quality, that is unlawful and constitutes usury, because he has made a loan stipulating an increase in quantity or quality. If he advances them in return for fewer items or something of inferior quality, he has merely forgone the greater quantity or superior quality to secure a guarantee of repayment. None of this is permissible, because the Messenger of Allah, peace and blessings be upon him, prohibited a loan yielding a benefit.
Only when the qualities differ clearly and substantially do the items resemble two distinct categories, taking the transaction beyond the character of a loan into that of a lawful sale. It then becomes permissible to advance one in exchange for the other, since either category may sell readily while the other remains unsold. This cannot occur within a single category. Since it cannot occur there, someone advancing an item in exchange for its like is certain of the benefit he has stipulated, so the transaction is impermissible. This differs from two distinct categories: when one is advanced in exchange for the other, neither the party making the advance nor its recipient is certain of obtaining the benefit he seeks, since either category may sell readily while the other does not.
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As for advancing money in exchange for money—gold for silver or silver for gold