"I heard the Messenger of Allah say: 'If the two parties to a transaction disagree, and neither of them has any proof, then it is as the owner of the goods says, or they may cancel it.''' (Using translation from Nasāʾī 4648)  

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «إِذَا اخْتَلَفَ الْبَيِّعَانِ وَلَيْسَ بَيْنَهُمَا بَيِّنَةٌ فَهُوَ مَا يَقُولُ رَبُّ السِّلْعَةِ أَوْ يَتْرُكَا»  

"We were with Abu 'Ubaidah bin 'Abdullah bin Mas'ud when two men who were involved in a transaction came to him. One of them said: 'I bought it for such and such', and the other said; 'I sold it to him for such and such,' Abu 'Ubaidah said" 'something like this was brought to Ibn Masud, and he said; I was with something like this was brought to him. He told the seller to swear an oath, them he gave the purchaser the choice; If he wished, he could buy it, and if he wished he could cancel (the transaction)'' (Using translation from Nasāʾī 4649)  

حَضَرْتُ رَسُولَ اللهِ ﷺ أُتِيَ فِي مِثْلِ هَذَا فَأَمَرَ الْبَائِعَ أَنْ يَسْتَحْلِفَ ثُمَّ يَخْتَارَ الْمُبْتَاعُ فَإِنْ شَاءَ أَخَذَ وَإِنْ شَاءَ تَرَكَ