"When you make a promise to the Lord your God, don't be slow to pay all that you promised. Why? Because the Lord your God will demand that you pay it. You will sin if you don't pay the things you promised.
But you must do the things you say you will do. If you make a special promise to God, then you chose to make that promise. God did not force you to make that promise. So you must do the thing you promised!