Congress sets the budget not the president. The administrations budget is aspirational, and if they want to force it they are required to use political savvy and whatever influence they have built up. Yeah so zero influence as all of that is towards cover ups, stock manipulation, and incompetence.
Technically true, but the president also selects the administrator of NASA, and this presidency isn't terribly fussed about following laws about how allocated money can be used.
So don't be surprised if suddenly half the NASA budget is used to pay for a second ballroom or more missiles to CENTCOM.
Yes, but this President has decided that he can move money around or just not spend the money, regardless of the budget, and this Congress has let him.
The executive has the veto and a willingness to leave the government non-functional (funny how anti-government types are often okay with kneecapping government). They're not powerless.