Again model the original dirham as 100 imaginary silver units. The changer supplies 49 units of silver plus fulus conventionally sold for half a dirham. A legally permissible allocation is:
49 silver units from the original → 49 silver units received
Remaining 51 silver units from the original → copper fulus
The same-metal component is now 49 for 49. The remaining 51 units are exchanged for a different kind of property, where that same-metal weight-equality rule does not apply.
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How the second formulation preserves silver equality
Bab al-Sarf: Circulating Fulus, Obsolete Coins and Money-Changer Exchanges
Again model the original dirham as 100 imaginary silver units. The changer supplies 49 units of silver plus fulus conventionally sold for half a dirham. A legally permissible allocation is:
49 silver units from the original → 49 silver units received
Remaining 51 silver units from the original → copper fulus
The same-metal component is now 49 for 49. The remaining 51 units are exchanged for a different kind of property, where that same-metal weight-equality rule does not apply.