Is a financial accountant's objection to the disparity in salaries between him and other less experienced employees in the same company considered a religious sin, knowing that he cannot terminate the contract before its term without losing his rights?

You must honor the contract with the company, as Allah the Almighty says: "O you who have believed, fulfill [all] contracts." It is permissible to request a salary increase, following the example of your colleagues. If the company agrees, then all praise be to Allah. If it does not, then you must adhere to the contract until its termination.