In congressional nomenclature however a budget is a guideline for appropriations bills, not something that carries the force of law. And Congress is quite speci

In congressional nomenclature however a budget is a guideline for appropriations bills, not something that carries the force of law. And Congress is quite specific about what it appropriates money for, even if it does it all in a several thousand page bill at the last minute. So a federal agency like the FAA generally prepares a budget recommendation that would be submitted to Congress as part of the president's budget, but then Congress would decide how much (if any) to appropriate for each activity, down to the level of individual programs in many cases. By the time Congress gets done with it becomes a list of appropriations, and the agency has no authority to transfer things around, spend money on what Congress did not authorize, or even purposely refuse to spend money (or spend less without a good reason) on what Congress has appropriated funding for.