The president can't decide to stop spending money Congress has passed by EO.
Now I do not know if Congress has explicitly required X to provide certain thing. If they did then there might be some issue if X fails to provide it. It's probably easy to say someone is accountable for it if it's something objective (let's say building for Congress) but if it's subjective it could get tricky.
This is called impoundment and it's unequivocally illegal.
The Trump administration will repurpose those funds to things more aligned with GOP preferences.