Musannaf Ibn Abi Shayba 13:763

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Mughira, from Ibrahim, who said: Every *hawala* (transfer of debt) is returnable, except if a man says to a man: "I sell to you what is owed by so-and-so for such-and-such." So if he sells it, then it is not returnable.