Qasim bin Abdur Rahman narrates from his father: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) sold a government slave to Ash'ath. There was a disagreement between them regarding the price. Sayyiduna Abdullah said: "I sold him to you for twenty thousand." Sayyiduna Ash'ath said: "I bought him from you for ten thousand." So Sayyiduna Abdullah said: "If you wish, I will narrate to you a hadith of the Prophet (peace and blessings be upon him), which I heard from the Prophet (peace and blessings be upon him) himself." So Sayyiduna Ash'ath said: "Please narrate it." So Sayyiduna Abdullah said: I heard the Prophet (peace and blessings be upon him) say: "When there is a dispute between two parties in a transaction, and the sold item is still in its original condition, and neither party has any proof, then the statement of the seller will be accepted, or they may both cancel the transaction." So Sayyiduna Ash'ath said: "I think you should cancel this transaction."