You don't really have to say anything to the dog owner. I'm also skeptical he plans to pay you; if he does, fine, if not, it's up to you whether you approach him again for the money or not. Personally I wouldn't bother because the hassle would not be worth it to me, and likely wouldn't net me any money anyway, unless I REALLY wanted hassles and went to court, etc. As for talking to him any more about leash laws, they were your pets, etc., to me it would be a waste of breath and not worth it. If you're supposed to send him a bill, make a copy of it then mail it to him. Certified.
Can't help you with guns. When I need one I call my son, he brings me what I need, we go over the safety etc., then I shoot. I'm a good shot but know little about the different kinds of guns. Sometimes I keep a shotgun around, as when fox were getting my chickens.
Another case of, shoot the owner, oops that's illegal, shoot the dog instead. So sad.