What is the ruling on one who takes the decisive oath, denying the plaintiff's right, relying on offsetting due to an amount owed to him by the plaintiff that covers the claim, fearing that the plaintiff would take it unjustly?

The defendant must be truthful in their oath before the judge; otherwise, they fall under severe warning. However, if the defendant has a right with the plaintiff and strongly believes the plaintiff will deny it, then it is permissible for the defendant to deny the plaintiff's right to the extent of their own right, even by oath, provided they intend ambiguity (tawriya) and not the literal truth. This is the view of the Malikis and Shafi'is, who cite the verse: "And the recompense of an evil deed is an evil deed like thereof" (Quran 42:40). But if there is no strong belief that the plaintiff will deny the defendant's right, then denial is not permissible. It is better to be truthful and avoid ambiguity, acting upon the principle of forgiveness and tolerance.