iPhone trade in question

If i trade in my xr for a 13, and the value of the xr is at £160, if i send it to apple and they value it at less than £160 can i send my new phone (the 13) back and get my xr back to get a better deal elsewhere? or is it a once i’ve payed and traded in i cannot get my xr back and have to pay the full price minus whatever it is valued at?

[Re-Titled by Moderator]