I thought you could return your phone before the end of your Next agreement without paying the remaining installments?
Here is this directly from
https://www.att.com/shop/wireless/next.html
AT&T Next 24: Divided into 30 installments; trade in and upgrade after 24 installments.
AT&T Next 18: Divided into 24 installments; trade in and upgrade after 18 installments.
AT&T Next 12: Divided into 20 installments; trade in and upgrade after 12 installments.
AT&T Next with down payment: 30% of device cost down, divided into 28 installments; trade in and upgrade after 12 installments.
I would consider that a lease option. Sure you can also lease-to-own if you continue payments.