According to the law, Congress is able to use its power to investigate and to oversee any operations of the many agencies in the executive branch.
This means that if the Congress determines that a federal program that is proposed by the white house is beyond conceived budget, they have the power, legally, to reject it in a way that it can't be passed and therefore, it won't be established. On the other hand, the President has the power to veto and with it, the president can pass some of these rejections