They did not start concrete planning for the use of the atomic bombs until April and May 1945. So by that point they were already quite sure that the bombs would not be ready until early August anyway, and that the Germans would not be around as enemies by that point. So there were never any plans to use it against the Germans.

The closest one gets to an indication that such a thing might have happened if the bombs were ready much earlier, is that Roosevelt had asked General Groves in December 1944 whether the bomb might be made ready to use on Germany at that point. The answer, because the bombs were not anywhere close to being ready for use at that point, was "no." But it is an interesting and rare indication about Roosevelt's frame of reference on that. But it does not tell you what kind of target they would have used it on; again, until May 1945 they did not even cement the strategy they were planning to employ (i.e., targeting cities, esp. ones that had not already been bombed), and that was done in the context of Japan in particular.