It was narrated from Qasim bin 'Abdur-Rahman from his father that 'Abdullah bin Mas'ud sold one of the slaves from the stateil to Ash'ath bin Qa is , and they differed concerning the price. Ibn Mas' ud said. "I sold him to you for twenty thousand," but Ash'ath bin Qais said. "I bought him from you for ten thousand." 'Abdullah said. "If you want, I will tell you a Hadith which I heard from the Messenger of Allah P.B.U.H " He said. "Tell me it." He said: "I heard the Messenger of Allah P.B.U.H say: 'If two parties to a transaction differ, and they have no proof, and the sale item remains (unredeemed), then what the seller says is valid. Or they may cancel the transaction," He said: "I want to cancel the transaction," And he cancelled it. (Hasan)