That the Messenger of Allah ﷺ said: "When the two parties (in a deal) disagree then the seller's statement is taken as valid, and the purchaser retains the option."  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا اخْتَلَفَ الْبَيِّعَانِ فَالْقَوْلُ قَوْلُ الْبَائِعِ وَالْمُبْتَاعُ بِالْخِيَارِ