When applying for part-time jobs

When hiring a part-time legal adviser of a limited liability company, the latter agreed to carry out this work if, taking into account his qualifications and experience, he will be paid under an employment contract in an amount exceeding the full salary stipulated by the LLC staffing for this position. In this regard, the question arose of the validity of such a condition, since under the current legislation on combining jobs, the working hours of a part-time worker may not exceed 4 hours per day and half of the established working time per month.
The lawyers of the company believed that payment, in this case, should be made in proportion to the time worked. The person accepted for the job concurrently believed that the remuneration of each employee should depend on the personal labor contribution, the quality of work and should not be limited to any maximum amount.
Who is right in this dispute? Is it possible to establish individual conditions for remuneration of labor under an employment contract with a part-time job?
What are the features of the regulation of labor part-time workers.