Is it permissible for a company owner to seal a construction contract for a fee, knowing that the contractor who uses his seal will pay a bribe to syndicate employees, and does he incur sin as a result?

Selling your brother his company seal is not permissible, due to the cooperation it entails in deception and fraud. However, if his money is mixed, there is no harm in benefiting from what he gives you, such as covering the costs of 'Umrah, according to Sheikh Qalyubi's statement: "It is not forbidden to eat, nor to deal with, nor to accept charity or gifts from someone whose money is mostly unlawful, except for that which is known to be strictly forbidden." But if all his money is unlawful, then it is not permissible to deal with him, nor to accept his gift. If you have your own money, it is better to perform 'Umrah with it to increase the reward.