Is it permissible for an employee to take money for completing transactions after official working hours, knowing that he does not fall short in his work during official working hours?

The employee of any governmental or non-governmental institution is a hired worker for them, and he must adhere to the tasks assigned to him by virtue of the contract concluded between the two parties. If his employer permits him to perform additional work, he may do so, and he is entitled to receive payment for his additional work. However, he must be careful not to fall short in his original work.