The way I see it, Dean has two goals when it comes to Sam, and neither conflicts with what Billie said. His primary goal is to keep Sam alive. This is more important than saving people and hunting things, as evidenced by his asking Sam not to complete the last trial.

But if he fails at that, his secondary goal is to not survive Sam's death. And again, this is more important than saving people. When he thought Sam was dead, he was ready to die fighting the other werewolves. He only changed his course when Corbin begged him to save him and Michelle. (And if you don't agree he was going kamikaze at that point, keep in mind that he could have said "you'll be fine; hide in the basement while I kill these guys." But he didn't, because that wasn't his intention. He had to change his entire plan in order to survive so he could save them.)

So, his offer to Billie was to save Sam outright (ostensibly to save the world), or take him in trade for Sam. Either option would meet one of those goals. Having Sam around to fight Amara was just a bonus as far as he was concerned.

(ETA... Hope I don't sound argumentative. I'm having a grand time discussing this.)