Whoa; he loses his life span the more he uses magic? Yeah...I wouldn't want his help anymore. Just tell the king that making gold for him once should be enough to prove herself & let her go o.o No amount of jewelry equals to him losing his life bit by bit...
I like this twist. I mean not that I like him literally losing his life using magic, but it would make sense that an ability that allows you to do impossible things would have a very high price.