Ironically, this is the check in the balance of the three branches. The Congress and The President must agree on a budget. It's sad that political game theory supersedes common sense planning ahead, but turning it around to "forcing will" mischaracterizes the propriety of the system. One day the US will evolve into a proper parliamentary system, but that should be shortly after we get instant binding political polls and have people born on mars (give or take 100+ years).

Anything he wants to spend on a wall, take from the military. If you're gonna spend it, shuffle it.