MDM removal

I just sold my old iphone which I bought direct from Apple.

New owner finds MDM block when trying to set up. After spending a day going round in circles I get confirmation from Apple that I owned the phone. IT Administrator of MDM Company is now investigating.

Apple Tech reckons they had an input error which put MDM on phone.

Given the new owner can't set up the phone, how will the IT Administrator be able to remove the MDM lock when they confirm it shouldn't be there?

Is there usually a "once off" user name and password they can give to disable it?