Is it permissible to sell a "resale" property to a client transferred from the company and collect commission, knowing that the company has a special department for that and the client can be transferred to it?

It is not permissible for an employee to convert a company's client to his own private business, because custom prohibits him from dealing with the client for his own account. Furthermore, the employee acts as an agent for the company in sales, and an agent is a trustee who must act within the bounds of his agency in a manner that is most beneficial to his principal.