Oooh..tricky one, dude.

I'd give the chance to explain himself, first - don't fly off the handle just yet...
If his explaination or remedy (ie, he'll give you what you paid for, regardless of his mistake) is not satisfactory, then the only option is to feed him his own arm

Either that, or vote with your feet, count your loses for paying up-front, and learn the lesson..harsh, but that's life, dude.
I hope it all ends amicably for you