Is it a rented or leased tank ? or does he own it ?
When our company started charging me rent for low usage (because of a mild year and new owners)I told them to come and get the tank. They reimbursed me for what was left in it.
If nothing else don't have them come get it yet. I'd haul it over to your place set it next to your tank and then hook it up and run it about empty then haul the tank back over to his place. Lift from the bottom or straps all around it and not the little hooks on top these are only for when empty.