Whoever gives wheat for wheat based on value, such as giving half a sa' of good wheat for a sa' of average wheat, it is not permissible; because it is not permissible to give part of what is specified for part of another based on value, whether what is given is of the same type or of a different type, as long as it is specified. Value is only considered in matters that are not specified. See: Bada'i' al-Sana'i' 2: 72-73.