What is the meaning of the Hadith from Ma'mar ibn Abdullah that the Prophet, may Allah's prayers and peace be upon him, said: "Food for food, like for like"?

The authentic hadith (definition) in Sahih (definition) Muslim is a fundamental proof for the obligation of equivalence when selling food of the same kind, and it prohibits any excess. Ma'mar differed from others on the issue of selling wheat for barley with a disparity; while he viewed it as impermissible, the Hanbalis, Shafi'i, Ishaq, and the People of Opinion permitted it, because wheat and barley are two different kinds.