Model the original dirham as 100 imaginary weight units. The first request allocates:
First 50 units of silver → copper fulus
Remaining 50 units of silver → 49 units of silver
The second correspondence exchanges 50 units of silver for 49 of the same metal, which creates the unlawful excess. The units are purely illustrative, not a claim about historic dirham or grain weights.
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Why the first formulation contains unequal silver
Bab al-Sarf: Circulating Fulus, Obsolete Coins and Money-Changer Exchanges
Model the original dirham as 100 imaginary weight units. The first request allocates:
First 50 units of silver → copper fulus
Remaining 50 units of silver → 49 units of silver
The second correspondence exchanges 50 units of silver for 49 of the same metal, which creates the unlawful excess. The units are purely illustrative, not a claim about historic dirham or grain weights.