Sayyiduna Mu’ammar bin Abdullah (may Allah be pleased with him) narrates: He sent his slave with one sa’ of barley and said: Go and sell it, and whatever barley you get in exchange, bring it back. The boy went and obtained one sa’ and a little more barley. When he came to Mu’ammar and informed him about it, Mu’ammar said to him: Why did you do this? Go and return it, and only take an equal amount, because I heard the Prophet (peace and blessings of Allah be upon him) say:“When exchanging grain for grain, it must be equal.”And in those days, our grain was barley.