Was I wrong to ask for money?

I've been trying to find the owners of a female Neapolitan Mastiff for almost a month and yesterday I posted a ad on craigslist and within a hour or 2 I got a reply from a gentleman saying he owned the dog. He sent me a few pictures of him with the dog and the pics do match the dog in question. I explained to him that if he wished to collect the dog then I would need vet receipts and $100. I'm not trying to hold the dog at ransom but I need to be sure that this is his dog, also the dog was pregnant when I found and her puppies are almost 4 weeks old. Meaning I have been taking care of both mom and pups for almost a month.