A judge sentenced the activists but said she did not want to discourage protest.
Curator Susanne Meyer-Büser says that New York City 1 (1941) has been upside down since it was first publicly seen.
The unnamed piece from the “Smoke over the Rooftops” series, was discovered on the flip side of Léger’s Bastille Day.
The work, titled ‘Madonna of the Magnificat’, will be offered by Christie’s this November.
Sotheby’s with offer the lot titled Composition No II in modern art evening sale in New York on November 14.
The painting, which hasn’t been authenticated, could be worth “millions of dollars.”
The work, recovered last September, is one of five Old Master paintings stolen in 1979 from Schloss Friedenstein.
The city-owned Stedelijk Museum had held the painting since 1940.
The culprit sold fakes of works by Richard Hambleton and Barkley Hendricks.