I say, and with God's success: Whoever extracts wheat from wheat based on value, by giving half a sa' of good wheat for a sa' of average wheat, for example, it is not permissible; because it is not permissible to perform what is specified partially for one another based on value, whether what is given is of the same kind or of a different kind after it has been specified. Value is only considered in what is not specified. See: Bada'i' al-Sana'i' 2: 72-73, and God knows best.