Do any of you offer exclusives to a merchandiser on your sites? If so, what kind of arrangements have you made with the merchant?

I have two sites that an existing affiliate merchant wants exclusivity on. They want me to do the seo and they will give me their content for use. I do not like the idea of promoting my sites for one merchant without some kind of monthly???income or some kind of guaranteed money. I would have to change about 50 pages to accomodate them.