I heard Abu Sa'eed Al-Khudri say: "The Messenger of Allah (ﷺ) said: 'Transactions may only be done by mutual consent."'
Sunan Ibn Majah Book 12, Hadith 49
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"The Messenger of Allah (ﷺ) bought a load of fodder from a Bedouin man. When the transaction was concluded, the Messenger of Allah (ﷺ) said: 'Choose (either to go ahead or to cancel the transaction).' The Bedouin said: 'May Allah grant you a long life of good transaction!"'
Sunan Ibn Majah Book 12, Hadith 48 Next Hadith → Abdullah bin Mas'ud sold one of the slaves from the state to Ash'ath bin Qais, 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 (ﷺ)" He said: "Tell me it." He said: "I heard the Messenger of Allah (ﷺ) 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.
Sunan Ibn Majah Book 12, Hadith 50
Sunan Ibn Majah Book 12, Hadith 48 Next Hadith → Abdullah bin Mas'ud sold one of the slaves from the state to Ash'ath bin Qais, 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 (ﷺ)" He said: "Tell me it." He said: "I heard the Messenger of Allah (ﷺ) 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.
Sunan Ibn Majah Book 12, Hadith 50