The budget becomes a law, once passed and signed by the President.
Can Trump ask Congress to not fund USAID in forthcoming fiscal year? Of course. But that conversation is passed to Congress as part of the budget. Trump can veto the proposed budget.
But he cannot unilaterally terminate appropriations that were previously enacted.
Otherwise, it would be fiscal chaos. Government projects (to include contractors and employees) would never have any reliance in moving forward.