Is the salary that a doctor receives for his work, which is equal to the salary of master's degree holders while his qualification is a bachelor's degree and a medical specialization diploma, permissible or impermissible (halal or haram)? And what should he do if he continues to work?

The wage must be known at the time of contracting for the lease to be valid; otherwise, it is فاسدة (invalid), and a fair wage (أجر المثل) becomes obligatory. If the hirer increases the employee's wage without fraud, there is no impediment, but it is stipulated that the increase must be with the permission of the one authorized to dispose of it. If it is from someone not authorized and you become aware of it, it must be returned to the employer. If that is impossible, then it should be given as charity for the benefit of Muslims. The lease contract must be concluded, fulfilling its conditions, including knowledge of the wage and the duration.