If Congress passes a spending bill, does the President actually have to use the money?

[Massacure]

If Congress passes a bill allocating $1,000 for Project A, $2,000 for Project B, and $3,000 for Project C, can the President refuse to spend some of that money? What if he doesn't need it. Like say Project B is for flu vaccines, and they cost $1.99 each. Does he have to buy 1005 vaccines? Can he round it off to 1000? What if he decides that he only wants 500 or any at all?

[Captain Hilts]

1. Yes. Nixon tried that and they outlawed not spending what was allocated. nt