Even if Heisenberg made the bomb first, it wouldn't be too long until Oppenheimer did the same.
The good thing for America was that they were far away from the action, Germany wasn't. Germany's attempts at making the A-bomb irl were halted by the Norwegians in Operation Gunnerside for example.
A-bombs don't come for free, most of the uranium for the manhattan project came from the shinkolobwe mine in congo. I can see Germany getting some jews to mine pitchblende but allied bombs would try fucking over any enrichment facilities.