He promised months back that he would challenge the impoundment law. He claims it's unconstitutional.

See: https://www.npr.org/2024/11/26/nx-s1-5195528/trump-impoundme...

So presumably he wants this to go to the Supreme Court. If the court rule against him, then Congress's authority is preserved, so I doubt they will impeach him. If the court rules in his favor, then I doubt they'd impeach him over something the court agrees with him on.