Musannaf Ibn Abi Shayba 13:1441

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Husayn, from Muhammad ibn Zayd, who said: I said to Ibn Umar: "A man might pay in advance to another man a thousand [dirhams] or the like, and he says: 'If you give me wheat, then it is for such-and-such [price], and if you give me barley, then it is for such-and-such [price].'" He said: "He must specify for each type of them a named amount of silver (dirhams). Then, if he gives you that which is in it, [take it]; otherwise, take back your capital."