Narrated Abdullah ibn Mas'ud:

Muhammad ibn al-Ash'ath said: Al-Ash'ath bought slaves of booty from Abdullah ibn Mas'ud 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-Ash'ath 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.

حَدَّثَنَا مُحَمَّدُ بْنُ يَحْيَى بْنِ فَارِسٍ، حَدَّثَنَا عُمَرُ بْنُ حَفْصِ بْنِ غِيَاثٍ، حَدَّثَنَا أَبِي، عَنْ أَبِي عُمَيْسٍ، أَخْبَرَنِي عَبْدُ الرَّحْمَنِ بْنُ قَيْسِ بْنِ مُحَمَّدِ بْنِ الأَشْعَثِ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، قَالَ اشْتَرَى الأَشْعَثُ رَقِيقًا مِنْ رَقِيقِ الْخُمُسِ مِنْ عَبْدِ اللَّهِ بِعِشْرِينَ أَلْفًا فَأَرْسَلَ عَبْدُ اللَّهِ إِلَيْهِ فِي ثَمَنِهِمْ فَقَالَ إِنَّمَا أَخَذْتُهُمْ بِعَشْرَةِ آلاَفٍ . فَقَالَ عَبْدُ اللَّهِ فَاخْتَرْ رَجُلاً يَكُونُ بَيْنِي وَبَيْنَكَ . قَالَ الأَشْعَثُ أَنْتَ بَيْنِي وَبَيْنَ نَفْسِكَ . قَالَ عَبْدُ اللَّهِ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ " إِذَا اخْتَلَفَ الْبَيِّعَانِ وَلَيْسَ بَيْنَهُمَا بَيِّنَةٌ فَهُوَ مَا يَقُولُ رَبُّ السِّلْعَةِ أَوْ يَتَتَارَكَانِ " .