Al-Muwatta Hadith 31.81

Malik related to me that he had heard that Abdullah ibn Masud used to relate that the Messenger of Allah, may Allah bless him and grant him peace, said, "When two parties dispute about a business transaction, the seller's word is taken, or they make an agreement among themselves. Malik spoke about someone who sold goods to a man, and said at the contracting of the sale, 'I will sell to you provided I consult so-and-so. If he is satisfied, the sale is permitted. If he dislikes it, there is no sale between us.' They made the transaction on that basis. Then the buyer regretted before the seller consulted the person. Malik said, "That sale is binding on them according to what they described. The buyer has no right of withdrawal, and it is binding on him, if the person whom the seller stipulated to him, permits it." Malik said, "The way of doing things among us about a man who buys goods from another and they differ about the price, and the seller says, 'I sold them to you for ten dinars,' and the buyer says, 'I bought them from you for five dinars,' is that it is said to the seller, 'If you like, give them to the buyer for what he said. If you like, swear by Allah that you only sold your goods for what you said.' If he swears it is said to the buyer, 'Either you take the goods for what the seller said, or you swear by Allah that you bought them only for what you said.' If he swears, he is free to return the goods. That is when each of them testifies against the other."