I'm sorry to hear what has happened to you, but unfortunately this is not extortion or griefing. Since he didn't play any part in helping you or encouraging you to make the mistake, there's probably not anything that can be done. MMOs are ripe with stories of folks who have lost gil or items by making mistakes such as yours, and you should consider yourself to be extremely lucky he returned as much of it as he did.

Certainly it would be nice of him to return all the gil, but there's unfortunately there's nothing that requires him to do so.