Bayhaqi 10438 — The buyer and seller have the option as long

Abu al-Wadi' says: We camped at a place during one of our military expeditions, and a companion of ours sold a horse in exchange for a slave. (Then the two of them stayed for the rest of that day and the night.) When morning came and it was time to set off, he got up to saddle his horse and regretted (the sale). He came to the man and asked to undo the sale, but he refused to undo it and said: "Sayyiduna Abu Barzah (may Allah be pleased with him), the Companion of the Messenger of Allah (peace and blessings be upon him), will judge between me and you." They came to Sayyiduna Abu Barzah al-Aslami (may Allah be pleased with him) in a corner of the army camp. They related the story, and Sayyiduna Abu Barzah (may Allah be pleased with him) said: "Are you both content that I judge between you with the judgement of the Messenger of Allah (peace and blessings be upon him)?" He (peace and blessings be upon him) had said: "The buyer and the seller have the option to cancel the deal as long as they have not separated."
(b) Hisham bin Hassan says that Jamil related that he said: I do not think that you two have separated.