All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The company undertaking to pay the bills of medical treatment of its employees is considered a service and benefit that some companies offer and condition –as you know –that the employee gets sick and provides real bills that he paid for his treatment.
Therefore, it is not permissible for an employee to provide forged bills in order to get a refund from the company because his right on the company is treatment and not the money itself.
Hence, you should repent to Allaah and give back the money to the company even indirectly, and if you are not able to return it, then you should spend it on the general welfare of the Muslims.
Allaah Knows best.