Peridot is right. With two demons, negotiations become a lot more finicky. Neither Jasper or Lapis can properly can work out a deal for fear that the other might undercut it.

Which means that when the other demon doesn't agree to the terms laid out right away, Lapis jumps on them, without really considering or thinking about it. She doesn't want to give Jasper a chance.

"I accept your terms." She states, letting the magic bind her to the human. "I think my powers could be used for grander things, but if that's what you want from me, so be it."