It is narrated from Sayyiduna Ma'mar bin Abdullah 'Adawi (may Allah be pleased with him) that he sent a slave to take one sa’ of wheat, sell it first, and then buy barley in exchange for it. So the slave went and brought back more than one sa’ of barley. When he came to Sayyiduna Ma'mar (may Allah be pleased with him) and informed him about the transaction, he asked: Did you really do it this way? Go back and return this transaction, and take only equal for equal in exchange, because I heard the Messenger of Allah (peace and blessings be upon him) say that when selling grain for grain, it must be equal for equal. At that time, our food was barley. Someone said: But these are not of the same kind. However, Sayyiduna Ma'mar (may Allah be pleased with him) said: I fear that it may resemble it (i.e., riba/usury).