Musannaf Ibn Abi Shayba 13:3210

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from his father, from Hammad, from Ibrahim, regarding a man who says: "If you do such and such, then for such and such [payment], and if you do such and such, then for such and such [payment]." He said: There is no harm in it regarding hiring (ijarah).