Broad brush stroke here, but if Congress doesn't allocate funds specifically, doesn't that give the executive branch a lot of leeway in distributing those funds?
There are different levels here.
First, there are specific earmarks, where Congress says spend X dollars on program A. A lot of the big ticket purchases are done this way. For example, Congress might earmark a set number of funds for a specific weapons system, or for a particular infrastructure program. The President has no choice. He has to spend that money.
Second, there are more generic allocations which aren't earmarked for specific recipients, but still specify an amount of funding that must go to a particular initiative. The President (or the particular agency in charge of the funds) will have discretion on exactly where to spend them, but they will still have to spend them. So, if Congress says "Spend $150M on bridges in Iowa," the Executive will get to decide which bridges get built and where, but they do have to spend the full $150M, and it has to go to bridges in Iowa. I believe this is the kind of allocation for which the Impoundment Control Act gives the President a method for requesting a drawdown of allocated funds, but it's only a request, and Congress has to approve it. If they don't, the funds must be spent.
Third, Congress has the authority to authorize but not require certain funding for some things. So, they might say, "Hey, Prez, you are allowed to spend up to $150M on research for new contraceptives for gerbils." The Executive isn't required to spend that money. The $150M is a limit, not a floor. Congress very rarely allocates funds in this way, but if they did, the President would in fact have the power to simply ignore it and not spend that money at all.
