A "dumb question" on the way. We have three branches of government with dedicated duties. You are right; congress doles out the money, but even that is with input from the executive branch since the executive branch is essentially the operations branch with budgetary needs for actions and staffing. After all how would congress supposedly know what a three letter branch coming under control of the president would need if not for input from the executive branch. Without doubt, the originators of our government understood how unwieldy congresses would be as an operations branch and delegated that to the executive branch - first hint: the name. So on to the question. Why would you supposed that with that delegation of authority, the founding fathers would have not given the chief executive (CEO) the authority for day to day governmental operations to include timing and go "no go" decisions?
A funky little example of an executive vs a bureaucracy. Let's suppose a congress delegated $50B in spending for the government of Alamania, and the just before the check went into the mail, Alamania was overthrown by an unfriendly group. Do you still think the president should abide by congressional mandate and send the money to a sworn enemy of the US? If we are arguing about withholding money for Ukraine, the why aren't we arguing about withholding F-35s to Turkey, and why did we not have this kind of "crisis" when we stopped F-14 parts to Iran years ago? I'm pretty sure we all know the answers and how politics perverts good ole common sense.