Is one sinful and subject to expiation if they swear on the Quran to return to a job they intend to leave, in order to secure their rights, knowing that they have not yet sworn?

If your employment contract has ended and your manager withholds your entitlements, and you find no way to reclaim your right except by swearing to return to work, then it is permissible for you to swear with tawriya (ambiguity). For example, you can intend by "returning" to resume earnestness and activity. If you are compelled to swear without tawriya to reclaim your money, it is permissible, because necessity permits the prohibited. Some scholars have permitted a false oath for a predominant benefit, as in the hadith of Umm Kulthum bint 'Uqba regarding reconciliation between people, war, and the hadith of the spouses.