This guy is a name to watch. He has an upcoming book listed on Amazon.com, "Rescued by the Reich: How One of Hitler's Soldiers Saved the Lubavitcher Rebbe". Not yet published but you can pre-order or put it on your wish list. The story sounds fascinating and he did some deep research.
I've heard this story before, but according to the version that I heard, the Rebbe was captured by the NKVD (the Soviets) who had a Jewish agent.
Not sure which version is true. My son in New York is an editor for Kehot publishers and has access to a great deal of "classified" information which is kept by the Hasidic sect and is not published. He might know the truth about this anecdote.