See, maybe I'm just not that nice, but I would have said that if he wanted the ring back he could grow up and come get it himself. Otherwise I would have just kept it as payment for the many more times you'll probably clean up their dogs poo. OR I would say that he'd get it back after they proved they dewormed the dog. It's one thing not to clean up the poo, but it's a whole other thing to not clean up wormy poo. Other dogs can get that! Not cool. I'm no genius, but I have a hard time with stupid people who have no intention to learn.
Good job on being the bigger person. It takes a lot for me to be that person.