Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever lets a remorseful person cancel his sale, Allah Almighty will forgive him on the Day of Resurrection."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11129