Is the additional salary received by an accountant for his work as a real estate agent, without the knowledge and approval of the employer, considered unlawful (haram) or lawful (halal)?

It is not permissible for an employee to take an additional salary without the knowledge of the employer, because the fundamental principle is to fulfill the trust of those who entrusted you. Also, because the right is not clearly established, if this were permitted, it would open a great door to evil. However, it is more befitting for the employer to appreciate the employee's efforts and reward them.