Musannaf Ibn Abi Shayba 13:893

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Hisham, from al-Hasan, he said: If you enter into a Salam contract for something, do not take anything except that which you entered into the Salam contract for. And do not make a Salam contract for something, then convert it into something else.