"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.'''  

سَمِعْتُ رَسُولَ اللَّهِ 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)''  

حَضَرْتُ رَسُولَ اللَّهِ ﷺ أُتِيَ بِمِثْلِ هَذَا فَأَمَرَ الْبَائِعَ أَنْ يَسْتَحْلِفَ ثُمَّ يَخْتَارَ الْمُبْتَاعُ فَإِنْ شَاءَ أَخَذَ وَإِنْ شَاءَ تَرَكَ