Is it permissible to keep the remainder of my car's fuel allowance, or must it be returned, knowing that if I were to use the car, nothing would be left of the money?

This depends on the system of the employer. Is the money disbursed considered ownership for the employee, so that the remainder belongs to him? Or was it paid for the expenses of the trip and the completion of the work, in which case the remainder must be returned unless the employer permits its use? For clarification, the employee can inquire with the employer.