Is it considered usury for me to pay for a device for a friend, on the condition that he repays me in installments with an increase, given that I purchased the device before selling it to him in installments?

If the questioner's friend is unaware that the questioner is profiting from him, this is not permissible, because the wealth of a Muslim person is not lawful except with his willing consent. Similarly, it is not permissible for the questioner to buy the device and pay for it in cash, then sell it to his friend for a higher deferred price, because this falls under the meaning of a loan that brings benefit. It is permissible for the questioner to buy the device for himself and take possession of it, then sell it to his friend through an immediate or deferred sale. Either the questioner informs his friend that he has bought the device and sold it to him in installments, or he should not take from him more than what he paid.