find my iphone turn off

If you no longer have access to a device you can remove a device from Find My on iCloud.com. --> Remove a device from Find Devices on iCloud.com - Apple Support --> "You can use Find Devices on iCloud.com to temporarily remove a device from the Devices list or turn off Activation Lock on a device you already gave away or sold."

- See the section "Turn off Activation Lock on a device"

Not sure I am following this. You did follow the instructions to return the phone, correct?

What to do before you sell, give away, or trade in your iPhone ...Official Apple Support › en-us

If not, you could try to remove the device.......www.icloud.com. Log on using your Apple ID and Password and see if you can remove the phone from the list of devices.