I think it depends on how you buy it.Are you sure you only paid the balance? If you ordered your iPhone through apple with a trade in, you did not just paid the balance, you paid the full price upfront.
Apple charges the full price upfront, then they give you a credit for the trade in when they receive it. If you don’t send the trade in device, they already charged your full price.
I bought both my phones on the 24-month zero interest Apple payment plan. I was given instant trade-in credit on both of them, and that was what the total cost/monthly payment was calculated on. If the trade-in isn't accepted, or I failed to send it in, the balance would be applied and my monthly payments adjusted accordingly.
I bought my AWU2 outright - that one was full price up front, and the trade in credit would be applied when they received my trade-in (which, it turns out, I'm not going to do).