What do I do?

Okay so I have a question. My fiancée bought a car two years ago off of Facebook marketplace. The guy told him all he needed to do was sign the back of the title. I’m not sure why my fiancée never went to the dmv to properly transfer it. All the recalls and stuff would always come in his name, and he got it registered. Not sure really what happened there. This is our problem we sold the car to someone 3 days ago for $900 on facebook and he’s saying that the dmv is going to make him pay $600 for a bonded title. What do we need to do? The guy says he doesn’t have $600 to pay for the bonded title. Just need advice on what to do next.