Is it permissible to take the price difference when selling a mobile phone to a friend at a lower price, knowing that this friend usually does not yield profit, and is it obligatory to inform him of this, or is taking this amount forbidden?

If your friend commissions you to buy a device at a specific price, and you buy it for a lower price, it is not permissible for you to keep the difference for yourself without his knowledge. This is because an agent is not entitled to profit unless the principal permits it. Therefore, the excess amount belongs to the principal.