Really, the President and his management of all of the various alphabet agencies isn’t part of the spending process in Washington?
I mean, when did Congress get back to deciding exactly how each dollar is spent? Because from my experience and short history on this earth, I have seen a dollar amount issued in the name of something or a department, but how it is actually spent is decided by each alphabet agency! Good example, Congress approved the Defense budget, but Trump lowered the cost of several defense weapons systems - do you think that money went back to Congress for additional spending discussions? NO!
Otherwise, we would already have a wall, since that was voted on by Congress many, MANY times.
The actual texts of laws that spend money can vary, and they are very important, as many court cases make clear.
The example of the 55 mph speed limit is instructive. It was a federal law passed by Congress that authorized withholding highway funds for non-compliance.