It was narrated from Qasim bin 'Abdur Rahman from his father that :
Abdullah bin Mas'ud sold one of the slaves from the state[1] to Ash'ath bin Qais, and they differed concerning the price. Ibn Mas'ud said: "I sold him to you for twenty thousand,' but Ash'ath bin Qais said: "I bought him from you for ten thousand." 'Abdullah said: "If you want, I will tell you a Hadith which I heard from the Messenger of Allah ﷺ" He said: "Tell me it." He said: "I heard the Messenger of Allah ﷺ say: 'If two parties to a transaction differ, and they have no proof, and the sale item remains (unredeemed), then what the seller says is valid. Or they may cancel the transaction." He said: "I want to cancel the transaction." And he cancelled it.

حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، وَمُحَمَّدُ بْنُ الصَّبَّاحِ، قَالاَ حَدَّثَنَا هُشَيْمٌ، أَنْبَأَنَا ابْنُ أَبِي لَيْلَى، عَنِ الْقَاسِمِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِيهِ، أَنَّ عَبْدَ اللَّهِ بْنَ مَسْعُودٍ، بَاعَ مِنَ الأَشْعَثِ بْنِ قَيْسٍ رَقِيقًا مِنْ رَقِيقِ الإِمَارَةِ فَاخْتَلَفَا فِي الثَّمَنِ . فَقَالَ ابْنُ مَسْعُودٍ بِعْتُكَ بِعِشْرِينَ أَلْفًا . وَقَالَ الأَشْعَثُ بْنُ قَيْسٍ إِنَّمَا اشْتَرَيْتُ مِنْكَ بِعَشْرَةِ آلاَفٍ . فَقَالَ عَبْدُ اللَّهِ إِنْ شِئْتَ حَدَّثْتُكَ بِحَدِيثٍ سَمِعْتُهُ مِنْ رَسُولِ اللَّهِ ـ ﷺ ـ . فَقَالَ هَاتِهِ . قَالَ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ ـ ﷺ ـ يَقُولُ " إِذَا اخْتَلَفَ الْبَيِّعَانِ وَلَيْسَ بَيْنَهُمَا بَيِّنَةٌ وَالْبَيْعُ قَائِمٌ بِعَيْنِهِ فَالْقَوْلُ مَا قَالَ الْبَائِعُ أَوْ يَتَرَادَّانِ الْبَيْعَ " . قَالَ فَإِنِّي أَرَى أَنْ أَرُدَّ الْبَيْعَ . فَرَدَّهُ .