Musnad Ahmad Hadith 27250

It is narrated from Muammar (may Allah be pleased with him) that once he gave one of his slaves a sa’ (a measure) of wheat and said, "Sell it and buy barley with whatever money you get." The slave went and brought back a sa’ and a little more, and informed Muammar (may Allah be pleased with him) about it. Muammar (may Allah be pleased with him) said to him, "Did you really do this? Take it back and return it, and only make an equal-for-equal transaction, because I heard the Prophet (peace and blessings of Allah be upon him) say that food should be exchanged for food in equal measure, and in those days our food was barley." Someone said, "This is not its equivalent." He replied, "I fear that it is similar to it."