An article, "Would the atomic bomb have been used against Germany?" (if, say, the bomb had been ready in December, 1944), discusses possible answers, including a conversation with General Groves years later, and some discussion in the comments of what available plane could carry such a load.
IMO, if the bomb and a delivery system were available in December, 1944, it would have been an act of treason to refuse to order the bomb dropped on Germany.
The crews of the 509th were drawn from the ETO for a reason. The intent was to use it on Germany.
But VE Day happened two months before Trinity.
The problem with using the bomb in Europe is that it would have crossed into other parts of Europe and killed a lot on non-Germans.
The “super secret, pinpoint accurate” Norden bombsight was propaganda. We ought to know by now that it takes, from the WWII POV, "sufficiently high technology to be indistinguishable from magic" to achieve the “pickle barrel” accuracy in level high-altitude bombing of which the Roosevelt Administration boasted. Simply too much unknown, or poorly known, in the problem.Why was the propaganda important? Because American voters circa 1940 considered British-style nighttime carpet bombing to be a war crime. Thus, American daylight raids using magically accurate bombsights were a political imperative in 1944. The Dresden raid was a blatant exception - and there were American bomber pilots who protested, and had to be intimidated into complying with the orders.
FDR was bloody minded about “unconditional surrender” - which was popular with civilians, but less so with professional soldiers because it was perfect for NAZI propaganda. Churchill didn’t like it either, but he was the junior partner and bit his tongue. It certainly didn’t reduce the fanaticism with which the Germans defended the Fatherland.
In any event, it would have been FDR, not Truman, making the call. And FDR would have wanted to do whatever was good for the USSR. But even though there was reciprocal racism at work in the military relations between Japanese and Americans, I come down on the side that you couldn’t actually hold back a war-ending technology at a time when the Allies were taking casualties at a horrific rate. It would have been interesting to see how FDR would have squared the circle of absolute rejectionism of Germans vis a vis surrendering to Russians and his commitment to the Russians not to negotiate a separate peace with Germany. Even with the A-bomb . . .