Is it valid to impose an oath on the plaintiff when there is evidence of the debt and the plaintiff is absent abroad?

The judge is more knowledgeable about the proceedings of the matter. It appears that your client is the defendant in the new lawsuit. The burden of proof is on the plaintiff. If the plaintiff fails to provide it, and there is a presumption for the claim, the defendant can refute it with an oath, based on the Prophet's (peace be upon him) saying: "The burden of proof is on the claimant, and the oath is on the one who denies." The judge's ruling resolves the dispute.