What is the ruling if, in an exchange of goods (like currency exchange or sale), one party discovers the received item is adulterated with a substance foreign to its nature?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
If the exchange was of the same substance for the same substance (e.g., gold for gold), the contract is void (fasid). If the adulteration is discovered in the session (before separation), and the recipient returns it to take back the original item, the contract remains valid because the exact object of the contract is being recovered. If they separate before returning the adulterated item, the contract is also void because they separated before taking possession of the intended substitute item.
Supporting text
A narration from Ahmad suggests that if the replacement item is taken within the session for returning the adulterated item, the contract is not nullified, analogous to cases where the defect is of the same kind as the item itself. If the buyer knew of the adulteration when purchasing an item with a defect of the same kind, the sale is valid, and there is no option for rejection or exchange.