Are the funds I received from the company considered a debt upon me, and how can they be repaid, and is it permissible to observe voluntary fasts before making up obligatory fasts?

The amounts taken from the treasury without permission are established as a debt and are not discharged except by returning them directly to their rightful owners, if possible, or indirectly otherwise. If the exact amount is not known, then it is obligatory to investigate and be cautious so that one's conscience is clear before Allah, along with repentance and seeking forgiveness.