Is it permissible to buy foreign currency from friends at a price lower than that of a merchant, or at the same price, in order to achieve personal benefit and benefit for them, in my capacity as the one responsible for providing the best price for the company?

If you are responsible for purchasing items for your company, you are not permitted to take the price differences for yourself, because you are an agent for the company. Any increase gained by an agent belongs to his principal, and he must work for the benefit of his principal. If purchasing from your friends is more beneficial for the company, there is no harm, and the difference belongs to the company, not to you.