“I once asked abu Ja‘far (a.s.), about the case of a man who gives foodstuff to a flour man under a contract that requires him to return for every ten Artal (plural of ritl, a certain unit of measurement) twelve Artal of flour. He (the Imam) said, ‘No, it is not permissible.’ I then asked about sesame given under a contract to an oil man who guarantees for every Sa‘ a certain Ritl (a certain unit of measurement). He (the Imam) said, ‘No, it is not permissible.’”