The Troll gave Daniel up because it was intrigued by Callie's kindness to a stranger[edit | hide | hide all]
Walter Lambert's son had already been trading with the Troll, likely for years on end, without ever getting what he wanted. My guess is that the reason why it gave Callie Daniel is because Callie showed kindness to Walter even though she didn't even know his name. Walter's son was trading to get back his father because he's family, so it's a give in. Callie didn't even know Walter, but she still showed kindness that the Troll misinterpreted as trading for his freedom, which impressed the faery.