How can expiation be made for an excessive sum of money taken by mistake five years ago, given that it cannot be returned for fear of exposure, and its owner has repented to Allah?

The money taken by mistake must be returned to its owners, who are the bank and the employee who made the error in handing over the amount, because the bank penalizes the employee at fault. Restoring rights is a condition for the validity of repentance, and the disgrace of this world is easier to bear than the torment of the Hereafter.