He can't enter your premises without your permission so removal of the exhibit would have to be in a manner agreed by you. If necessary you can get a court order to keep him off your premises. So he isn't really in a position to dictate to you how this is done.
You have offered access. They ant entitled to take access as may put your property at risk.
No they can't insist on this if there is another way more convenient and safe for your property. It's your museum and your property therefore what you say goes, basically.
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