What is the ruling on the salary earned after a promotion in a job based on a forged certificate?

What your brother did, forging a certificate to get a promotion at work, is forbidden because it involves lying, falsification, and taking a salary he is not entitled to. What is built upon falsehood is false. Therefore, it is not permissible for him to take the increase after a promotion based on forgery. He must repent and return to his original position or leave the job, and strive to obtain a genuine certificate that qualifies him.