By now, it's pretty simple to find different ways of refusing invitations in Japanese. However, what I can't really seem to find is a reply to a refusal. Something like I'm sorry you can't make it, I hope you can join me next time or even a simple That sucks, man.
So, what would be an appropriate reply to somebody's ちょっと用事があるんですけど (or whatever)? The only thing I could come up with my limited Japanese skills is something like 残念ですね but that seems a bit bland.
I'd be interested in both a casual version and a more formal one.