Congress passes the budget and spending bills. That's where all the taxes get decided, and all the spending gets decided. The executive branch is supposed to manage the organizations that go and spend that money as allocated.

Suppose Congress passes a budget that includes $100 Billion for tanks for the army. The army says it doesn't need them, but the money will go to a district with a powerful congressman so it ends up in the budget. Now, the President KNOWS that in six months the budget is going to run out of funding without a raise in the debt ceiling because the Congress passed the budget saying the money could be spent but didn't raise the debt ceiling enough to cover it.

Can the President tell the Pentagon to HOLD on buying those tanks? (answer is NO, apparently, see below)

There are many things just like those tanks in the budget, so assume I'm also talking about anything else he believes is either waste or lower priority. Meanwhile he continues or even accelerates spending on the social programs he believes are critical, so that when that debt ceiling comes around the funding for his social stuff is what's already been spent but the funding for the things he considers waste has not? There are many ways to do this aside from just saying "Don't buy the tanks". For example, the Executive branch could add so much red tape to the process, or force more competitive studies, delay decisions on final vendors, or any of a thousand other tricks to delay the spending.

The Constitution gives Congress the power of the purse strings. It is the job of the Congress to pass a budget every year. Without congress allocating funds, nothing gets spent. The Debt Ceiling fiasco comes because Congress allocates more money to be spent than that which comes in, increasing the debt, but then doesn't authorize the debt itself to be increased without coming back to Congress first for another negotiation. They've increased their own power. I'm wondering if by carefully allocating which funds are actually spent and when, the President can take some of that power back.

Update (thanks to Bob for giving me the right term to search on):

Wikipedia has a great summary: http://en.wikipedia.org/wiki/Impoundment_of_appropriated_funds

The Congressional Budget and Impoundment Control Act of 1974 was passed in response to perceived abuse of the power under President Nixon. Title X of the act, and its interpretation under Train v. City of New York, essentially removed the power. This severely inhibited a president's ability to reject congressionally-approved spending.

The Impoundment Control Act of 1974 provides that the president may propose rescission of specific funds, but that rescission must be approved by both the House of Representatives and Senate within 45 days. In effect, this has removed the impoundment power, since Congress is not required to vote on the rescission and has ignored the vast majority of presidential requests.

Forty-three U.S. states give their governors the authority to not spend money allocated by the state legislature. The states which deny their governor the authority are Indiana, Maryland, Nevada, New Hampshire, North Carolina, Rhode Island, and Vermont. The Mayor of Washington, D.C. also has this power.