What is the primary basis for the ruling concerning claims?

Primary text

The fundamental basis for the ruling on claims is the Prophet's saying, reported by Muslim: "If people were given solely based on their claims, some groups would claim the blood and property of others, but the oath belongs to the defendant." Another narration states: "The evidence is upon the claimant, and the oath is upon the defendant." Furthermore, a claim is only valid if issued by a person legally competent to dispose of property.