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 Himself will forgive him, that is, He will forgive his mistake."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11131