“Once abu ‘ Abd Allah (a.s.), was asked about the case of a man who sells foodstuff to another man in quantity of several kur but cannot deliver what he owes, and says to the other party to accept two Qafiz (a certain unit of measurement) of barley for one Qafiz of wheat to compensate for the deficit. He (the Imam) said, ‘It is not valid; the origin of barley is from wheat, however, he can return his dirhams proportionate to the degree of deficit.’”