Trade it
Is it possible to cancel a trade in ... after purchasing a new iPhone
Question marked as
Yes. See the following from https://www.apple.com/shop/trade-in
If you haven’t yet shipped your device, you can cancel your trade-in by simply keeping your device.
If you’ve already shipped your device, the trade-in can’t be canceled. If you receive a revised trade-in value after the inspection of your device, you can then choose to reject it.