He will run away and give you nothing. NOTHING!!

I've heard if you use a non-damage dealing skill like Steal, he will fight you without using Comet, but I've never tried. Comet wasn't so bad anyways.

If I recall correctly, he doesn't give the greatest amount of points. But giving him a Diamond is just kind of... pointless. There are better things to do with your Diamond than give it to him and have him skip town.