iPhone 15 Pro Max trade-in confusion with Apple Finance and Barclays

Hello, I have an iPhone 15 pro max that I purchased through the Apple Finance with Barclays. I have purchased a new iPhone 16 pro max and I chose to trade the 15 pro max in. The pop up in the app asked if I wanted to use the value to close off the balance of the loan to which I replied no, because I wanted to continue paying it off with the instalments. I went in store and they didn't accept the old phone for the trade in cause they said it needs to be paid off entirely first. I'm confused as to how this is linked? I still am liable for the rest of the outstanding balance and shouldn't be that between the bank and I? If I still have a standing order with Barclays and they still get the money every month, why wouldn't the store take the old phone?

[Re-Titled by Moderator]