It is narrated from one of the sons of Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When the two parties to a sale disagree and there is no witness between them, the seller will be asked to take an oath; then the buyer will have the option: if he wishes, he may take it, or return it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10809