Just a thought, but are you positive you are speaking of Nassau, Germany and not Nassau, New York?

Since you say he was first found to your knowledge in New York City in 1850 and Nassau is also a county of New York that borders on New York City it was just a thought that someone may have jumped to a conclusion from only the word "Nassau" in a note somewhere (just as I've seen "Magna" become "Magna, Utah" instead of "Chew Magna, England" from one word and a place name autofill in the same family tree you are researching).