Is the amount paid by the seller for bringing in customers considered a permissible commission, given that the price of the computer does not increase because of it and remains consistent with market prices?

Your maternal uncle's situation is one of two: either he is an agent for the client, and agency is built upon trust, and the client has entrusted him by appointing him as an agent. In this case, he must act in the best and most beneficial way for the client, and it is not permissible to take a commission except with the permission of the client (the principal buyer).