I found a little bit of an exploit; because the RNG for the merchant isn't saved it is possible to visit one and if you don't like what they are giving you, reload the page and visit again to see some different items. I haven't tested it but the same might be true for fight rewards. Also I think that you should only be able to talk to Matthew if you make it to floor 3. Otherwise you can do the same thing until you get what you want from him.
Edit: This is possible for all events, but will not change what a ? is on the map, that stays the same.