Presidential Spending Discretion and Congressional Controls
Click on the second one down that has: {PDF} Spending Discretion and Congressional Control
It will download a PDF t your computer that is approximately 30 pages from a book. Haven't fully read the PDF but I am thinking it is up to the President to actually spend the money there or not.
There are also other links that look to be promising, also of PDF's, following the one mentioned above.
I don’t buy that executive discretion canard especially when it excuses liberal spending. Faithful execution of the laws does not entail entertaining a chief executive’s whim, but within the bounds of law. Same goes for Congress in its appropriation of money.