How do I get my money back from a Craigslist purchase?

My husband saw an ad on Craigslist for a 1 1/2 year old Rottweiler. They texted back and forth and settled on $350.00. We went to her house, met the dog and decided we would take her. My husband handed her $350.00 in cash. We had the dog less than 24 hours and decided it wasn't going to work because she didn't get along with our 2 cats. He contacted her, told her what happened and she said we could return the dog. When we got there we dealt with her fiancé who was very nice but told us he's on disability and needed the money to pay his electric bill. So he didn't have the money to give us back but he said once he got his next check he would send us the money. Well my husband kept checking Craigslist to see if they sold the dog. When he saw the ad was gone again he contacted her and very politely said if they sold the dog he'd like to get his money back. Now she's basically telling us we're not getting our money back. What can we do?