How can one repent and return an unknown amount of stolen money to the employer without informing him or losing the job? Is it permissible to repay it by working additional hours without pay, and how is the value of these hours estimated Islamically?

To absolve oneself of financial obligations to people, one must either return their rights to them or obtain their forgiveness and absolution. It is obligatory that the right reaches its owners by any means. The person is not required to deliver it to them in person or inform them of what they have done. One may seek pardon and forgiveness from those whose money was taken, even without informing them personally. The amount that must be returned is determined by the person's own diligence, caution, and estimation of what they believe will clear their conscience. Working extra hours for free is not sufficient compensation for money taken. It is preferable for the person to work and receive their salary, then return it to the employer indirectly.