Go into a corner where he can't see me to slip on the goblin ring.
Walk up to the goblin and say "Hello, fellow goblin, I'm here to relieve you of your duties. I have now come to watch over the treasure chest for you, being a goblin and all. You may go now, as I am totally a goblin and can stand guard just as well as you, because I'm a goblin. Did I mention that I was a goblin?"