Musannaf Ibn Abi Shayba 7:199

Yazid ibn Harun narrated to us, he said: Hisham informed us, from Hammad, from Ibrahim, who said: If a man says to a man, while he has a debt owed to him: "If you do not pay me by such and such a day, then it is charity upon you," then it is nothing. And if he says: "And if you do not give it to me by such and such a day, then it is charity for the poor," then it is as he said.