Musannaf Ibn Abi Shayba 13:504

Abu Bakr narrated to us, he said: Hashim ibn al-Qasim narrated to us, he said: Shu'bah narrated to us, he said: I asked al-Hakam and Hammad about a man who buys something from a man, and he says: "If it is for cash, then it is for such-and-such, and if it is for a deferred term, then it is for such-and-such." They said: "There is no harm in it, provided they depart upon one of the two." Shu'bah said: I mentioned that to Mughirah, and he said: "Ibrahim used to see no harm in that, provided they separate upon one of the two."