What is the ruling on money received by an employee from airline companies as a personal incentive for selling tickets, without the employer's knowledge, and what should he do if he has spent this money or invested it in a business?

If the commission is given to the employee in return for work that the employer or companies prohibit, then it is a forbidden bribe, and one must repent and give in charity what was acquired from it to good causes. However, if the commission is without the knowledge of the office owners and is not in return for prohibited work, then it is permissible as a gift or brokerage fee.