The owner of the painting is putting it up for auction. Nothing wrong with that.
The government has the option to bid on the painting, or to make an offer to the owner.
This is how the National Gallery acquired the only Leonardo Di Vinci painting in the Western Hemisphere in 1967. They bought it.
A billionaire should buy it and donate it to the National Gallery. MacKenzie Scott, for example. She is in the process of giving away multi-millions.