Is it permissible for me to take the discount I get from the merchant after negotiating with him, knowing that I do my best to get the best price for the manager?

It is not permissible for an agent to take the exchange rate difference for himself without the company's knowledge and permission because the agent must act in the best interest of his principal. It is not permissible for him to take anything from the price except with the knowledge and permission of the principal. Therefore, any increase the agent adds to the price or any gift the buyer gives to the agent due to the contract belongs to the principal.