Is it permissible to pay a sum of money to an employee in another company to complete a sale, knowing that refusing to pay exposes the employee to penalties from his company, and he cannot leave his job?

If a purchasing agent asks for a sum of money for himself in order to buy from your company, then if this is with the knowledge and permission of his company, there is no harm in it and it is considered a price reduction. However, if he asks for that without his company's knowledge, then it is a forbidden bribe that is impermissible to pay or engage in, and if your work necessitates such an act, then you must leave it.