Jim Yanik wrote:

I read the other day that Congress authorizes the expenditures,but the
Executive is under no -obligation- to actually spend the
money,particularly in the bill's extensions,which may not be
Constitutional.IOW,if an authorization is in a bill's extension,that
money could be withheld. AFAIK,no President has actually tried this
yet.

Not exactly correct. Many presidents have withheld final spending (Thomas
Jefferson was the first). Nixon tried "impoundment" of funds allocated by
Congress. He was sued and the courts ordered that he spend the appropriated
funds.

"In 1974, with Nixon's Presidency in its death throes, Congress passed the
Budget Control Act of 1974, which stripped the President of the power of
impoundment... it [the Budget Control Act] totally marginalized the
President as a major player in the game of budgetary politics, and the
budget went out of control."

However, it is the current president's position that hundreds, if not
thousands, of "earmarks" were dropped into the final bill without committee
hearings and, as such, are not properly part of the resulting appropriation.
Currently, the president has his legal staff looking into the matter.