If I sold the phone I would have to make sure it covered the rest of the iUP contract, if it didn't I would be out of pocket. Why bother when I can give it back to Apple and they will pay off the contract and let me set up a new one?
I'm not sure if it will work next time but with my IP7 I had something like £200 remaining to pay off. I sold the phone for like £350 paid off the remaining £200 and put the extra £150 towards the iPhone X
But yeah if that's easier for you I was just suggesting it