What is the ruling on a person swearing that the belongings his colleague took are not his, while he knows that they are his belongings?

Lying is fundamentally forbidden, based on the Prophet's (peace be upon him) saying: "Beware of lying, for lying leads to wickedness, and wickedness leads to the Fire." However, it is permissible when it brings about a religiously recognized benefit or repels harm, if that cannot be achieved except through it. The best approach is to use dissimulation (tawriya).