I think he would have let them walk knowing he had got the better of them. Why go through the charade of signing her over if he was just going to kill them? And he had the dogs rip him apart because Django kinda gave him the go ahead to show he wasn't soft. Leo's character was crazy good. That scene where he slammed his hand on the glass and cut himself and Tarantino told him to go with it was epic. The look and reaction of the girl when he rubbed his blood over her face was improvised and her horrified reaction was priceless.
I just don't think they were sure that Candy would give her up though.