This is not a definitive answer, bu I did not find any proof that the Nazis had prohibited any German scientists from keeping a Nobel Prize.
The story of how their medals were kept safe breaks into two parts: the first part is why they sent their medals to Denmark, and the second is why they were dissolved. There seems to be few years gap between the two events.
About the sources:
The best source I could find seems to be following : The case of the bottled Nobel medals.
This note was written fairly recently (the most recent source was published in 2007), and the author seemed to have had access to some unpublished correspondance between some of the protagonists of the story.
Sending the medals to Denmark. It is probable that Franck left his medal at the NBI (Niels Bohr Institute) when he was a visitor there, that is between 1933 and 1935. The date at which von Laue left his is however unknown. In any case it is probable that Frank and von Laue left their medals at the NBI during the affair surrounding Ossietzky’s peace prize (1935-1936).
However, note that Hitler only forbid German scientists to accept a nobel prize on January 30, 1937. That means that Franck and von Laue sent their medals because they feared that their medals may get confiscated (as opposed to because they would have been confiscated), either because of a general law, or because they could both be targeted as political opponents (Franck because he was Jewish, von Laue because of his vocal opposition to Nazism). I did not find any clue that a confiscation law was ever passed concerning Nobel Prizes.
Dissolving the medals. This part is better documented so here is what de Hevesy wrote: (emphasis mine)
”My work was interrupted only one day during the enemy occupation of Denmark. When, in the morning of Denmark’s occupation [9 April 1940], I arrived in the laboratory, I found Bohr worrying about Max von Laue’s Nobel medal, which Laue had sent to Copenhagen for safe-keeping. In Hitler’s empire it was almost a capital offense to send gold out of the country and, Laue’s name being engraved into the medal, the discovery of this by the invading forces would have had very serious consequences for him. (Three years later the invading army occupied Bohr’s Institute.) I suggested that we should bury the medal, but Bohr did not like this idea as the metal might be unearthed. I decided to dissolve it. While the invading forces marched in the streets of Copenhagen, I was busy dissolving Laue’s and also James Franck’s medals. After the war, the gold was recovered and the Nobel Foundation generously presented Laue and Franck with new Nobel Medals.”
In particular Bohr and Hevesy weren't afraid of the medals being confiscated, bit of the fact that exporting gold was forbidden.
I think it is improbable that a law confiscated nobel prizes was ever passed in Nazi Germany (a definitive answer may be in this article but I do not have access to the full text) . This does not mean however that no medal was confiscated, only that it was done on an individual basis. In particular there is the case of Ossietzky's medal. However there are two things that may me think that it wasn't confiscated:
- The first one is that he was already in jail by the time he was awarded the Nobel Peace prize, so it is unlikely that he ever received the medal.
- The second is that Shirin Ebadi is said to be the first Nobel Peace Prize winner whose medal has been confiscated by national authorities (wikipedia).