Hadith 3511
Narrated Abdullah ibn Masud: Muhammad ibn al-Ashath said: Al-Ashath bought slaves of booty from Abdullah ibn Masud for twenty thousand (dirhams. Abdullah asked him for payment of their price. He said: I bought them for ten thousand (dirhams). Abdullah said: Appoint a man who may adjudicate between me and you. Al-Ashath said: (I appoint) you between me and yourself. Abdullah said: I heard the Messenger of Allah ﷺ say: If both parties in a business transaction differ (on the price of an article), and they have witness between them, the statement of the owner of the article will be accepted (as correct) or they may annul the transaction.
Hadith 3512
Al-Qasim bin Abdur-Rahman reported on the authority of his father: Ibn Masud sold slaves to al-Ashath bin Qais. He then narrated the rest of the tradition to the same effect with some variation of words.