"You tend to get rewards for returning stolen property, not for trying to sell it."

He's just saying. Before he can either threaten Stan himself (not as likely) or try to talk him into handing it over freely (likely on his end, but he gets the feeling it would be like trying to get a chicken to learn magic,) Cullen is offering him something.

"But in the interest of ensuring nobody gets hurt, there's your reward," he says, holding his hand out for the ring.