Abdur Rahman narrates from his father: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) sold a government slave for twenty thousand, that is, to Sayyiduna Ash'ath bin Qais. He brought ten thousand, so Sayyiduna Abdullah said: "I sold you for twenty thousand." Ash'ath said: "I did this for ten thousand; I am content with your decision regarding this." So Sayyiduna Abdullah bin Mas'ud said: "If you wish, I will narrate to you a hadith from the Prophet Muhammad (peace and blessings be upon him) regarding this." He said: "Alright." Ibn Mas'ud said: The Prophet Muhammad (peace and blessings be upon him) said: "When two men make a transaction and neither has any proof, then the statement of the seller is considered, then both should annul the transaction." So Sayyiduna Ash'ath said: "I annul this transaction."