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 Muslim cancel his sale, overlooking his slip, Allah Almighty will cancel (his slip) on the Day of Resurrection, that is, He will forgive him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11130