Mishkat al-Masabih 2880

‘Abdallah b. Mas’ud reported God’s Messenger as saying, “When two people who are arranging a business transaction disagree the decision rests with the seller, but the buyer has the right to choose whether he will confirm this*.”
Tirmidhi transmitted it.
In the version of Ibn Majah and Darimi he said, “When two people who are arranging a business transaction disagree, the commodity being present and is neither being able to prove his case, the decision rests with the seller, or they may both reject the transaction.”
* The seller swears an oath to the effect that he is right. The buyer may either agree or swear in an oath that he is right. In the latter instance the qadi cancels the deal.