Trying to find the IMEI on this broken iPad

A bit of a complicated issue, but here's what I need:

We are a hardware reseller and distribute phones and tablets to several companies. We have this iPad 6th gen that we got from another reseller that turned out to have a MDM lock on it (this happens from time to time, not to big of a deal). We go to them asking for a return and we get the RMA process started.

Once the device is returned to them, they let us know that the device arrived broken. They told us that this was likely our customer's fault. They did not realize however that we never sent this out to the customer, rather I sent it back directly. I am 100% sure the package was shipped properly, as even they said the box it was packaged in had 0 damage. They shipped the iPad back to us, with the outside box we shipped it in. The box has the IMEI on it, but I am trying to determine if they swapped the tablets on us.

I record all IMEIs, so I know which one I sent them, unfortunately we did not record the Serial Number, which is on the back of the tablet. If so I can confirm this real easily.

TL;DR: How can I get the IMEI out of this? It is not on the back, or the SIM tray. It won't turn on so I can't plug it into iTunes to get that info either. Is there a way to match the Serial Number to an IMEI?

Thanks in advance for any help