The Special tax status for foreign executives in B (Brussels)

Hello, I’ve just obtained the “Special tax status” for foreign executives working in Belgium and I would like a tax advice on the changes (reduction) that I noticed in my payslip (Monthly gross salary: “Remuneration de base”) compared to what is stated in my employment contract.
From what I understand, the Employer determine the new “base salary” according to the guidelines issued each year by the Belgian tax authorities (the so-called “technical note”).
According to that guidelines the “base salary” in the country of origin is deemed to represent the salary that the expatriate would have earned if he/she would have stayed in his/her home country. It determined by the application, on the gross salary in Belgium, of an index (specific to that country) and divided by the expatriation premium (in principle 1.10, but 1.15 for overseas countries). The index is defined per country by the tax authorities and subject to changes.
Now, my questions are the following:
1.The correction of the base salary (reduction) is always done or the Employer can keep the same gross salary regardless of the expatriate's home country?
2.Is it a choice that belongs to the Employer?
3.The correction of the base salary (reduction) can be applied even if the Monthly Gross Salary is agreed between the employee and the employer and is stated within the employment contract?
4.It seems clear to me that:
a.in case of correction of the base salary (reduction), the benefit of the special tax status is more for the employer then for the expatriate.
Indeed, even if the expatriate would have a higher net salary than the one received without application of the special tax regime the greatest benefit is undoubtedly for the Company since there will be a reduction of the Gross salary paid to the Employee. Please confirm!
b.in case of same gross salary (no correction of the base salary), the benefit of the special tax status is for the expatriates since he/she would have an higher net salary. The employer, instead, would not benefit from any reduction of the Gross salary to be paid to the employee. Please confirm!