What is the ruling on the work and salary of one who succeeded through cheating and obtained a certificate, knowing that the exam cannot be retaken, and how can he atone for that?

If a student cheats on an exam, succeeds due to cheating, obtains a certificate, and works with it, then he must repent, seek forgiveness, and regret what has passed. Allah forgives, and "the one who repents from sin is like one who has no sin."