The ruling on selling poor wheat or barley for good wheat or barley with a difference in quantity

Question:
Honorable Shaykh, may Allah grant you success. If you sold ten kilograms of poor wheat or barley for ten kilograms of good wheat or barley, is this permissible, knowing that both sellers are in agreement?

Answer:
It is not permissible to sell a thing for its own kind, wheat for wheat or barley for barley, where one is more than the other because one is better or of higher quality. This is riba (usury or interest). The Prophet (peace and blessings be upon him) said: equal in measure. Even if the kind differs, from one ribawi item to another, increasing the quantity is not permissible. Even if they agree to the riba, it is not permissible. The criterion is not agreement; the criterion is what conforms to the Shari'ah.