If you got a gift receipt from your brother for the device, Apple will offer you an Apple Store Gift Card than returning the amount to the original payment method. You can use it to purchase a new device. However, Sept 12 is about 14 days from the allowed return window so it is possible they can do an exchange for a newer device.
Or be up front about it to your brother that new devices are about to announce in the coming days and return the device early to allow sufficient time for that amount to go back to the original payment method and purchase a new device on pre-order day.