Next contract return/purchase

i have an iPhone that was purchased last year on a Next installment contract that is due to be returned in 14 days to fulfill the contract balance. I have purchased a new iPhone also on a Next installment. I have decided I prefer to purchase the old phone and keep it as a backup. How do I go about
doing that? My ATT rep suggested I just don't return it and then I will be billed for the balance. Any input appreciated.