What is the ruling on the surplus money that the questioner will receive when selling his relative's device to his friend, knowing that he will first buy the device from his relative for a price higher than the merchant's price? And does the ruling differ if the questioner takes out a loan to buy the device from his relative and then sells it for a profit?

A distinction must be made between the buyer and the agent authorized to sell. The buyer acts in an original capacity with regard to the commodity he owns, and he may sell it for whatever price he wishes. As for the agent authorized to sell, he must consider the interests of his principal, and it is not permissible for him to conceal any part of the price or take anything from it except with the principal's permission.