What is the ruling on the salary received by someone who bought his academic certificate for promotion? Is it entirely unlawful, or only the increase? And am I sinful for issuing a fatwa that his entire salary is unlawful?

Purchasing certificates is a forbidden deception, and this has been detailed in previous fatwas. Money earned from a job obtained with a forged certificate is unlawful. However, a salary received for a valid certificate of a lower qualification is not unlawful. One must repent from issuing religious rulings without knowledge.