Busr bin Sa'id (may Allah have mercy on him) narrates from Sayyiduna Ma'mar bin Abdullah al-Adawi (may Allah be pleased with him) that he sent a slave of his with a sa' of grain and said: Sell it, then buy barley with it. The slave went and brought back a sa' and something more. When Ma'mar came, he was informed of this, and Ma'mar (may Allah be pleased with him) said: "Why did you do that? Go and return it; take nothing but like for like, for I used to hear the Messenger of Allah (peace and blessings be upon him) say: 'Food, like for like.'" And our food in those days was barley. It was said: It is not the same as it. He said: "I fear that it may resemble usury (riba)."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18219