It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the buyer and the seller disagree, and the goods are still present in their original state, then the word of the seller is accepted, or the two of them return (the transaction)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10327