What constitutional basis have presidents cited for refusing to spend congressionally appropriated funds? in what two types of situations have presidents typically exercised the option not to spend appropriated funds?

The answer as varied as the history of the President has lengthened. The employment act of 1946, The Budget and Accounting Act of 1921 and the Anti-Deficiency Acts of 1905-6 have all been cited as justifications for Presidential impoundment.

Thomas Jefferson is notable as being the first to exercise Presidential impoundment, as it related to the passage of funding for military ships which were no longer needed by the time the bill reached the Presidents desk. So Jefferson, impounded the funds because they were no longer needed for the reason they were appropriated or allocated.

Richard M. Nixon is notable as having been the last to attempt to exercise Presidential impoundment, as congress finally closed this option to Presidents. Nixon had attempted to use his powers to eliminate funding for groups/activities which he deemed contrary to his own beliefs/policies. He used Presidential impoundment in an attempt to subvert groups who he opposed ideologically.

Afine-grained igneous rock forms as a result of slow cooling.

White dwarf is very hot, but also very small and that is why it may appear dim.