Seems like a transfer of title to me. There could well be a recision claim by the original owners, who sold under duress, but there is usually a statute of limitations for such claims. Also, the law is applied as it existed at the time, or else it is an ex post facto law. That is all under US law of course. Who knows what German law provides. It does create some interesting issues, though, as to works that were legally purchased by a private German citizen from Jews who were fleeing Hitler and selling at rock bottom prices. As for art stolen by the Nazis from Jews who were being sent to the camps, those should be returned, no issues.
Now that so many have been recovered, there will surely be a feeding frenzy on an unprecedented scale as the heirs of the original owners, real or imaginary, call in lawyers in an attempt to recover such spectacular treasures.
Who can blame them? A single Picasso can be worth up to £60million. A Matisse £30million.
The list of German painters alone reads like the roll-call of a lost generation.
I believe the authorities are determining ownership...and they know many of these were Jewish Owned property and are currently trying to establish that ownership......but you do wonder how many died in the camps whose families may not have a clue about them....These were actually found in 2011...so already a couple years into this.