"I'm the elected president so I get to decide the allocation of government money" is basically the legal theory that Trump is putting forward to be challenged.

A plain reading of the Constitution as well as the 1974 Impoundment Control Act should both be enough to debunk that legal theory, but alas here we are.

[1] https://www.stevevladeck.com/p/120-the-impoundment-crisis-of...