Is it permissible to benefit from a contract obtained by paying a sum of money to a third party without the knowledge of the contracting company, and is it permissible to accept a personal sum of money from this third party, knowing that the company reduced the salary and canceled the due commission?

If the intermediary is a foreigner to the other company and mediates for a known fee, then he is entitled to the fee from your company, and this is separate from the contract value. Adding the intermediary's commission to the contract value after its conclusion and without the company's knowledge is forbidden. However, if your company calculates the fee and the contract value and then negotiates with the other company on the total of that, and they agree, then there is no harm.

It is not permissible for you to take a commission from the other company or the intermediary for your work in your company; your right is with your company. It is not permissible for the company to reduce your salary as long as the contract period is valid.

However, if this person is an employee of the other company and one of his duties is to conclude such contracts, then his request for something for himself from your company is a forbidden bribe, and it is forbidden to give it to him.