He got a forged certificate so that he could get a promotion and a better salary
What you brother did, forging the certificate in order to get a promotion at work, is a haraam action, because it involves lying, forgery and taking a salary that he does not deserve.
Whatever is based on that falsehood is also false, and it is not permissible for him to take the extra salary that was given to him after he was promoted on the basis of that false certification.
What he must do is repent to Allaah and go back to his old position, or leave that company and work to obtain a new certificate that will qualify him for the position he wants.
See also question no. 6418.
And Allaah knows best.