How can I clear an iPad from another company's DEP?

The company I work for is in the process of enrolling all iPads into MDM and correcting devices not in the DEP. We've found one that we've had for a while. It's in another DEP **but not** in another MDM... So we are prevented from adding it our DEP but we have nobody to contact about correcting it. We contacted Apple with no results. Nobody has a serialized proof of purchase, the device wasn't asset tagged properly whenever it was acquired. My best guess was that somebody independently bought a refurb unit from a third party seller. How can I go about properly cataloging this into our DEP?