Quduri applies the same prohibition of unequal exchange to predominantly precious-metal coins as to high-quality coins. Fath al-Qadir makes weight equality explicit for such exchanges.
For example, giving coins classified as ten grams of silver against eleven grams of pure silver is not justified merely by minor alloying: the legal genus remains silver.
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Pure versus slightly alloyed precious metal
Bab al-Sarf: Mixed Currency Exchanges, Debased Coins and Obsolescence
Quduri applies the same prohibition of unequal exchange to predominantly precious-metal coins as to high-quality coins. Fath al-Qadir makes weight equality explicit for such exchanges.
For example, giving coins classified as ten grams of silver against eleven grams of pure silver is not justified merely by minor alloying: the legal genus remains silver.