Congress controls the purse, not the President and his increasingly political departments. If they want to change the law, they should vote on a bill first.
Congress authorizes spending but the executive actually does it and, in practice, can fail to do it selectively. If not statutorily authorized, this can be illegal, but the fact that it's illegal doesn't mean a whole lot until someone uses that fact to force it to be undone, which can take quite a long time.
> If they want to change the law, they should vote on a bill first.
What if they just want to arbitrarily exercise power and don’t give a flip about the content of the law, what should they do then?