Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Prophet (peace and blessings be upon him) said: "If people were given (everything) on the basis of their claims, people would lay claim to the blood and the wealth of others; but the oath is upon the one against whom the claim is made."
Imam al-Shafi'i (may Allah have mercy on him) says: When two men transact the sale of a slave, and the seller says: I sold it for one thousand dirhams, and the buyer says: for 500 dirhams, the seller is claiming the higher price while the buyer is claiming the lower price; both will be made to take an oath, but it will begin with the seller.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10803