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If it was donated by someone, shouldn't they give it back to the person who donated it? How does it get sold for cash?

In short, because it was donated and not loaned. Once it leaves your hands, the museum can do as it wishes (within the restrictions of its bylaws).

“Philanthropic trust” is probably the concept you missed here. Don’t worry, it looks like Allen missed it too.

And with fairly unusual exceptions, museums don't tend to like a lot of restrictions on what they can do with donations.