So when the claimant asks that the defendant be put under oath, the judge administers the oath to him and then dismisses him, for the starting presumption is that the defendant’s liability is clear.
Yet the defendant’s oath is only valid on two conditions: it must match the exact wording of his answer to the claimant, and it must follow the judge’s instruction issued at the claimant’s request, because the right to demand the oath belongs to the claimant and is not exercised except at his behest.
If the defendant declines to swear and refuses the oath, judgment is entered against him for his refusal (nukūl). Had the claimant been lying, the defendant could have repelled the claim simply by swearing; once he refuses, that refusal becomes an obvious indication of the claimant’s truthfulness—one that overrides the presumption of innocence.
Ruling on the basis of refusal is the position of a number of scholars and was the judgment of ʿUthmān (may Allah be pleased with him). Another group maintains that the oath is then shifted to the claimant—especially when his side appears stronger.
Imām Ibn al-Qayyim (may Allah have mercy on him) said: “The Sacred Law stipulates that the oath is imposed on the party whose side is stronger; whichever of the two litigants holds the preponderance, the oath is placed upon him. This is the view of the majority, including the scholars of Medina and the jurists of hadith such as Aḥmad, al-Shāfiʿī, Mālik, and others.” He also said, “This is how the Companions ruled, and Aḥmad and others approved it,” and he added, “It is by no means far-fetched for him to swear and then collect what is due, and this was likewise the choice of the Shaykh.”
Abū ʿUbayd said, “Transferring the oath (radd al-yamīn) has a foundation in the Quran and the Sunnah.”
Shaykh al-Islām Ibn Taymiyyah (may Allah have mercy on him) remarked, “What has been transmitted from the Companions concerning refusal to swear and transferring the oath is not contradictory; rather, each of the two rulings has its proper place.”