Muwatta Imam Malik (Riwayat Yahya)

Imam Malik, may Allah have mercy on him, said: If someone objects that all types of qutniyah (legumes) are considered as one category for zakat, whereas in the chapter of riba (usury) they are considered as separate categories, because it is permissible to exchange two seers of masoor (lentils) for one seer of mash (mung beans) in cash, but wheat, however, is one category, as it is not permissible to exchange two seers of white wheat for one seer of yellow wheat. The answer to this is that the rulings of zakat and riba are not the same. See, gold and silver are combined in one place for zakat, even though in exchange for one ashrafi (gold coin), you can take several times more silver.