What is the ruling when currency exchange involves gold or silver adulterated with an impurity that compromises the equality (tamathul)?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
If the exchange is gold for gold or silver for silver, and the adulteration compromises the required equality, the exchange is void. An exception exists if the adulterated gold or silver is sold for an equivalent amount of the same adulterant, such as exchanging a counterfeit dinar for another one known to have the same degree of counterfeiting, which is deemed permissible.
Supporting text
If an adulterated item is sold for an unadulterated item of the same kind, the transaction is impermissible unless the adulteration has discernible value, in which case it follows the ruling concerning the sale of dates (a measure of capacity).