What should one do who took money from his work with the intention of returning it, but the employer has since passed away, and he is currently unable to repay the accumulated amount?

Repentance from a forbidden act must involve four things: abandoning it, regretting it, resolving not to return to it, and returning the right to its owner. Since the owner of the right has died, it must be returned to their heirs. If you are unable to repay it, ask them for forgiveness or a delay. Allah Almighty will help with the repayment if the intention is sincere.