Wiped Mac Air for return - company still claiming device is FMI

Hi

A Mac AIR was purchased as a present, unfortunately the model was wrong. This was only found out after turned on and transfer of data. Subsequently hard wipe and returned to company.

Company/seller are stating that the returned Mac Air is still logged into my FMI

A) It was a hard full wipe

B) Device has been removed from account - cannot be seen on account

Even though I could not see the device I then

logged all devices out from account and did a password reset

Company/seller again still stating it is logged/locked to my FMI

How is this possible and what can be done - they will not refund until this is done

Thanks