Firstly, you should seriously develop the habit of blowing off all offers of this sort. Ulterior motives, hidden agandas, and "rights" issues will always follow you and be unfavorable to you: keep what you have as yours, and merely "reward" the growing interest of others appropriately.
Install PAP (Post Affiliate Pro) on your website so that you can run your own "affiliate program" in selling your books and stuff. You can even create a separate page/area for this guy for a 're-labeled' book that is still yours but with his preferred packaging, so to speak, and offer a different price and even a different level of commission since he is driving all the traffic to this one product.
This avoids the "licensing" pratfalls Karen suggested (since novices should not venture in this area without full legal representation), as you are merely re-packaging your product, and setting particular commissions accordingly: if he is so interested in it, your "customizing" it for him and setting up a means to control the traffic for it with higher commissions should prove to be a similarly attractive arrangement for him (especially financially): if not, then you have the "proof" you need to better formulate a plan as how to best deal with the next proposal, yes?
Installing PAP will preserve your rights, allow you the liberty to modify your product accordingly (re-market), and award greater financial "partnerships" without encumberances, easily managed in your corner.