What is the Sharia-compliant action to take concerning a person who claims the existence of a guarantee (kafala) on merchandise, and has a witness, while the alleged guarantor denies it and requests an oath?

If two people dispute over a contract, such as a sale or a guarantee, the claimant is asked to provide evidence (bayyinah) to prove their claim. If evidence is presented, judgment is given in their favor, and the oath of the denier is not considered. However, if the claimant does not have evidence, the denier is asked to take an oath (yameen) that the contract did not occur. If they swear the oath, judgment is given in their favor, but if they refuse (nakala), judgment is given against them.

Among the acceptable forms of evidence for establishing a right is one witness along with the claimant's oath, where the oath substitutes for the second witness. So, if the claimant presents a just witness and swears an oath with them, judgment is given in their favor. If they do not have any evidence at all, the denier swears an oath that the contract did not occur, and no judgment is made against them.