I used to think that when Fletcher found out that his son wished he could be incapable of lying for 24 hours, they why could Fletcher unwish it. The reason Fletcher couldn't do it himself because it would only work on birthdays.
When Fletcher gets Samantha to admit that she lied about her age so she get married, he triumphantly shouts "AND THE TRUTH WILL SET YOU FREE!" at the top of his lungs. When you think about this a minute, you realize that's actually a fairly accurate description of what follows: However legally dubious it might be in Real Life, in the context of the movie, the truth set Samantha free from her contract.