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.  

عَبْدُ اللَّهِ إِنْ شِئْتَ حَدَّثْتُكَ بِحَدِيثٍ سَمِعْتُهُ مِنْ رَسُولِ اللَّهِ ﷺ فَقَالَ هَاتِهِ قَالَ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ إِذَا اخْتَلَفَ الْبَيِّعَانِ وَلَيْسَ بَيْنَهُمَا بَيِّنَةٌ وَالْبَيْعُ قَائِمٌ بِعَيْنِهِ فَالْقَوْلُ مَا قَالَ الْبَائِعُ أَوْ يَتَرَادَّانِ الْبَيْعَ قَالَ فَإِنِّي أَرَى أَنْ أَرُدَّ الْبَيْعَ فَرَدَّهُ