Device Registration Program – How to erase an iPad from the DEP of another company?

The company I'm working for is registering all iPads at MDM and fixing devices that are not in the DEP.

We found one we had for a while. It's in another DEP but no in another MDM … so we can not add it to our DEP but we do not have anyone to contact to fix it. We contacted Apple without any results. No one has proof of serialized purchase, the device was not properly labeled when it was purchased.

My best guess was that someone had bought a renovation unit independently from a third-party seller.

How can I correctly catalog this in our DEP?