Exchange of One Kind of Food Items for another Kind

14. A number of our people have narrated from Sahl ibn Ziyad and Ahmad ibn Muhammad from ibn Mahbub from ‘Abd Allah ibn Sinan who has said the following: “This is concerning my question before abu ‘Abd Allah عليه السلام, about the case of a man who makes an advance payment for oil to receive ghee. He (the Imam) said, ‘It is not valid.’”