That is correct, the budget includes an estimate of amounts that will be spent. However, before spending the money, each department must draw up a detailed plan, which must be approved by Treasury Board.
You are absolutely correct in saying you cannot exceed the amount of money that was allocated. From a legal perspective, it's at the vote level. Legally speaking, when Parliament approves the appropriation bill, they vote on capital, operating, and grants and contributions. A department cannot exceed the total voted by Parliament in those three categories. That's the legal thing that cannot be exceeded. Below that level, departments can move money between areas as they see fit.