Gold for gold: same genus and weighing standard, so equal weight and prompt possession are required.
Gold for silver: the genera differ, while both are weighed monetary commodities; unequal weights may be lawful, but deferment and failure to obtain the required sarf possession are not.
The example of 10 g gold for 100 g silver is not forbidden merely due to unequal weights; the delivery terms and remaining contract conditions must still comply.
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Riba: Operative Causes, Unequal Exchange and Deferral
Gold for gold: same genus and weighing standard, so equal weight and prompt possession are required.
Gold for silver: the genera differ, while both are weighed monetary commodities; unequal weights may be lawful, but deferment and failure to obtain the required sarf possession are not.
The example of 10 g gold for 100 g silver is not forbidden merely due to unequal weights; the delivery terms and remaining contract conditions must still comply.