Originally Posted by kdmccaskill
...should i make the owner sign a legally binding agreement that he will not enforce the copyright, or should i just buy the option
Yes and/or yes. If he drew some manga and told everyone they could do their own versions of it, that means one thing. (I actually helped a friend with a website who was working at a manga publishing company and fan art came up every day.) Saying, anyone can draw or do their own fan creations is one thing. Giving away the rights for a film or TV show is a whole different game that I can't imagine this artist meant. Maybe they did, but if anything ever got made and it made money, I'm sure a lawsuit of some sort would break out.
Northbank makes a valid enough point, thus definitely something to keep in mind. But, again, yes, get the rights before you write something. If you can get the "film & TV" rights specifically for free, then great. If not, offer a few bucks. Maybe do them as an option only if he seems to start "smelling money" and then set up some kind of purchase price in the agreement, in case something ever really happens.
But as noted by others, you better cover yourself legally
in every way you can. Get the rights secured in writing. Who knows? The artist truly might not care in the least, but I'd find that extremely hard to believe as would any studio or network.