Mukhtasar Muslim

Narrated from Sayyiduna Mu’ammar bin Abdullah (may Allah be pleased with him) that he sent his slave with one sa’ of wheat and said, “Sell this and bring back what you get.” The slave went and brought back one sa’ and a little more of barley. When he came to Mu’ammar (may Allah be pleased with him) and informed him, Mu’ammar (may Allah be pleased with him) said, “Why did you do this? Go and return it, and do not take except equal for equal. Because I heard the Messenger of Allah (peace and blessings be upon him) say: ‘Sell grain for grain, equal for equal.’ And in those days our grain was barley. People said, ‘There is a difference between barley and wheat (so excess should be permissible).’ But he said, ‘I fear that both may be considered of the same type.’”