Considering the cost of shipping, I wouldn't have him ship it back. I'd have him take it to amp repair shop in his area and offer to pay the bench fee to see what's wrong. Better yet would be for you to contact a shop in his area first, have it all arranged for him. That way, if it's actually working fine and he's just trying to get a lower price from you, his bluff will be called.
I'm not saying he's dishonest, sometimes things happen in transit, but it's always good to be a little wary of claims like this.