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.  

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  

Ibn Mas'ud sold slaves to al-Ash'ath b. Qais. He then narrated the rest of the tradition to the same effect with some variation of words.  

أَنَّ ابْنَ مَسْعُودٍ بَاعَ مِنَ الأَشْعَثِ بْنِ قَيْسٍ رَقِيقًا فَذَكَرَ مَعْنَاهُ وَالْكَلاَمُ يَزِيدُ وَيَنْقُصُ