He tells a lie, his nose grows, but if it does grow, he'd be telling the truth and it wouldn't grow, then he'd be telling a lie and his nose will g-....... あなた get the idea.... -____-
The nose would grow. Now (as an indicator of time) is vague and open to interpretation. The moment the word finished coming out of his mouth and a micro-second passed without the nose growing it would of course begin to grow in response to the lie.
My Nose Will Grow Now -> Doesn't Grow -> Which Causes it to Grow