If we can't fix it, you do not pay.
Wordpress themes these days are getting amazingly complex. That also means that there is more occasion where bugs can occur in themes. And sometimes they are very difficult to find.
You may have just purchased a Wordpress theme. And there may be bugs in it. I've seen (and fixed) on many occasions off-the-shelf themes that have bugs in them.
I've seen enough Wordpress theme bugs, that I am confident that I would be able to fix it. If I can not fix it, I do not get paid.