Nothing will happen. If anything does happen, say you didn't know. The worst that happens is you have to hand them another dollar and after seeing the stink you made the first time, they'll be sure to give you exact change. Also if the owner walks over while you're having this conversation with the girl its prime time to bring the situation up to him.
I'm a server, I know. I've had people who couldn't pay, I've seen people who left less than the tab and walked out, along with witnessing a whole host of other things.