What is the ruling on work and the salary obtained after successfully completing a doctorate, given that the success came after a professor helped raise the student's grade from a failing mark (59/100) to a passing mark (60/100) after a debate in the university department, and is this considered favoritism?

As long as the head of the department raised your grade based on his authority and within the established regulations of the college, and without knowing your person, then the certificate is valid and there is no deception in it. You may work with it or request a promotion based on it. The mere fear of favoritism from the professor does not affect this, unless it is confirmed.