Is it actually coming out of the military budget? I assumed we were just going to get taxed for it, or they'd pull it from some other places that actually needs the funds.
Alternatively, they could just increase the military budget by 300 billion (or more).
That's only part of it, the US and allies are reportedly going to set up a $300 billion fund for Iran, the particular terms of accessing it aren't clear at this point. GP is correct in that the US is not putting up $300 billion, but it will be putting up part of that money. The US fought and lost a war and Iran comes out ahead, and the US and its allies come out worse and poorer.
Alternatively, they could just increase the military budget by 300 billion (or more).