I wonder who really has legal ownership. Clearly, 3rd Reich individual "stole " from museums and then GI's "liberated some of those and were returned to the states. I wonder if the museum had records and if so, are they available or destroyed? Granted art from that period is still being discovered and returned to rightful owners but this one is tricky trying to establish rightful ownership.
I don't think the museum would be inclined to pursue it after all these years. What documentation did the auction house have as to where it came from?