Is money earned from overtime considered unlawful if the actual work hours are less than what is paid for, with the knowledge and approval of the direct supervisor?

The fundamental principle is the obligation to adhere to official working hours and for the salary to be commensurate with the hours completed by the worker, based on the generality of Allah’s statement: "O you who have believed, fulfill [all] contracts," and the Prophet’s (peace be upon him) saying: "Muslims are bound by their conditions." An exception to this is what has been permitted by the employer. If the direct supervisor is authorized to grant such permission, then there is no harm in leaving early with his permission.