Matt tweeted that he performed the day before with a fever..obviously he did not improve.
I have gotten tickets refunded to me on occasion for various reasons, including if an actor (who was not even the "star" was out). I think it depends on who you wind up talking to at the box office. Some are really nice and do what they can do..and if they can't do anything, at least state it nicely.
Recently, I had an extra ticket to Hand to God and asked if I could return it and the box office woman practically chopped my head off...and insinuated that if I tried to sell it outside the theater, she could have me arrested if I was anywhere remotely near the box office. But that's uncommon...