Why don't you email and ask for a good will bonus on your next $500 deposit to help make it hurt less.
I could see them agreeing to that. They are human, they will know you don't want to fork over $500 if you don't have to.
It would see them get their $500 and keep you as a customer. If I was them I'd give you 100% bonus with a rollover to do it personally.
There is nothing wrong with the book btw. They are one of the oldest around and solid as as far as payouts. And they didn't try to dick you around or trick you into paying it back. There is no need to be trying to drag them down over it.