What is the ruling on an employee who is assigned a task, and then performs his work as well as the work of others?

An employee is a private contractor. He is not permitted to work in any other capacity during official working hours except with the employer's permission, because this time belongs to the employer. It is not permissible for him to use it for anything other than what it was designated for, except with explicit or customary permission. If the additional work is part of the institution's work or for its benefit, there is no harm in it as long as he performs his original work, as this is considered a voluntary act of kindness.