Is exaggerating one's health condition to obtain extended sick leave considered fraud, and how can one absolve themselves if the answer is yes? Is it permissible to take what jurists call "seizing one's usurped right" from the company as compensation for its non-compliance with labor law, by resorting to deception to obtain extra sick leave?

The doctor's extension of the leave may not be due to an exaggeration of the condition, but rather to his review of the medical file and his desire for precaution. If you deliberately exaggerated in order to extend the leave, then it is considered deception and fraud. However, if the exaggeration was unintentional, then there is no sin upon you. If you are certain that the doctor extended the leave due to your exaggeration and that you do not deserve it, then you must return the income from that extra period of leave to the company by any means. As for compensation for overtime hours, if the company is subject to the aforementioned law and is delaying your right, then it is permissible to take it by way of "al-Dhafr" (seizing one's right), and that includes counting the income from the extra leave as part of your right.