This is true only to the extent that Congress delegates its power to the executive. Per Article I of the Constitution, Congress has the plenary power of the purse.
So if it decides to spend $X on something specific, it has to be spent on whatever that something is. The President doesn't have discretion in that case.
So if it decides to spend $X on something specific, it has to be spent on whatever that something is. The President doesn't have discretion in that case.