Is the exchange of two adulterated gold coins, where the degree or amount of adulteration differs or is unknown, permissible?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
The exchange is impermissible if an adulterated coin is exchanged for a like coin, and the adulteration in both differs or its amount is unknown, because this violates the intended parity.
Supporting text
If the equality of the gold content and the adulteration in both is known, the ruling follows the two established legal possibilities. The preferred view is permissibility because both items are equal in both the intended component and the unintended component, and this does not lead to unequal exchange upon valuation, as the adulteration is unintentional and thus considered valueless.