It is narrated from Muhammad bin al-Ash'ath (may Allah have mercy on him) that Sayyiduna al-Ash'ath bin Qays (may Allah be pleased with him) bought some slaves from among the slaves of the khumus (the one-fifth share) from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) for twenty thousand. Then Sayyiduna Abdullah (may Allah be pleased with him) sent to him demanding their price, and al-Ash'ath (may Allah be pleased with him) said: I took them for only ten thousand. Abdullah (may Allah be pleased with him) said: Then choose a man who will judge between me and you. Al-Ash'ath (may Allah be pleased with him) said: You yourself are (the judge) between me and yourself. So Abdullah (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When the seller and the buyer disagree and neither has any evidence, then what the owner of the goods says is to be accepted, or else the two of them cancel the sale." This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.