Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

from his father that : Abdullah bin Masud sold one of the slaves from the state to Ashath bin Qais, and they differed concerning the price. Ibn Masud said: "I sold him to you for twenty thousand,' but Ashath 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 (saw)" He said: "Tell me it." He said: "I heard the Messenger of Allah (saw) 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.