Is swearing falsely regarding national service an oath that entails a dive into sin (yameen ghamoos), and will it affect future sustenance and invalidate any job obtained? And what must a person do about that?

A false oath concerning a past event is a Ghames oath (a dipping oath that immerses the swearer in sin). It necessitates sincere repentance and the restoration of rights to their rightful owners. The majority of scholars hold that it does not require expiation (kaffarah), while Imam Shafi'i and some other jurists maintain that expiation is obligatory. Combining repentance with expiation is more cautious. This oath does not affect future sustenance, and whoever repents, Allah accepts their repentance.