Is the buyer sinful if he pays an inflated price for a product sold by an authorized dealer for a telecommunications company, knowing that its price is fixed by the company and the dealer is asking for more than that, and is this increase considered a bribe from the buyer?

The company either sets a specific price for its goods for the seller and authorizes him to take the excess as his commission, which is permissible. Or, it delegates him to sell and sets the price for him, but does not authorize him to take any excess. In this case, any excess belongs to the company, and it is forbidden for the agent to take it. If you know nothing about the agent's agreement with the company, there is no harm in buying from him at a price higher than the known price. However, if you know that the company did not authorize him to take the excess, then it is not permissible for you to buy from him with that excess, because that would be assisting in something forbidden, and assisting in something forbidden is prohibited.