Wouldn't the nazis use him till they got the A-bomb, then kill him?
Three problems:
1) The Nazis may well not believe he can get them the A-bomb. The going assumption was that Einstein's theorey was hogwash. Unless he could prove (and want to prove) he could do it, it's unlikely he'd be tapped.
2) It is nowhere near obvious that Einstein *could* have built the bomb. The man was a solitary, visionary genius, not a brute-force development engineer.
3) Given the resource base, the project would still be refining yellowcake by '45.
Much more likely the regime would fire Einstein, relegate him to second-class citizen status and evetually send him to the camps.