Is it permissible to purchase an iPhone 6 in installments with a known profit after the buyer has chosen the specifications and the seller has paid the device's price to the store?

If you have purchased the phone legitimately and it has entered into your possession, then it is permissible to sell it to a prospective buyer for the agreed-upon price, whether paid upfront or in installments. However, if you have not actually purchased the phone and it has not entered into your possession, then the transaction is not permissible, because it constitutes a usurious loan, which is forbidden, as you would be paying the price on behalf of the student and then recovering it from him in installments with an increment.