Sayyiduna Ma'mar bin Abdullah (may Allah be pleased with him) said: I sent the slave off with a sa' of wheat (and told him): Go, sell it, then buy barley. The slave went and took a sa' and something more. When Ma'mar (may Allah be pleased with him) came, the slave informed him. Ma'mar (may Allah be pleased with him) said: Why did you do that? Go and return it, and take only equal for equal, for I heard the Messenger of Allah (peace and blessings be upon him) say: "Food for food, equal for equal," and in those days our food was barley. It was said: But it is not of the same kind. He said: I fear that it may resemble it.
Ma'mar bin Abdullah (may Allah be pleased with him) disliked this out of fear of falling into usury — a precaution on his own part, not a narration.
The narration from the Prophet (peace and blessings be upon him) is general; it covers two possibilities: 1. that only a single kind is meant; 2. or that both (kinds) are meant together.
Therefore, since Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) settled one of the two possibilities, it is to that one that one must turn.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10507