I would sell it for a token figure (Like - domain cost) and then agree to receive a percentage of the profits for the next 12-24 months.
If you want to be fair this would be the way to do it. You don't lose out too much if it makes a craptonne and he does not lose out if it makes nothing.
If you want to be fair this would be the way to do it. You don't lose out too much if it makes a craptonne and he does not lose out if it makes nothing.

I guess its a bit of development work... you don't mean the FB Like button do you?