Why do you think Faust is lying? What sort of intentions or benefits do you think he'd gain from it?

My principle suspicion is that he's manipulating her to get her to follow a line of action that will set her up to do something. How he is giving her information, how he gets a reaction out of her and such points towards an agenda, a goal he wants to steer her towards.

My first thought is that he's just following a script that Jacques laid out for him. But Jacques and "script" doesn't fit together much.

I suspect that he is saving her for his own ends that he isn't disclosing. Or not saving her at all, merely delivering her to another party who would make use of her and what she'll become (if she wil become what he says she will).

But the fact that he introduces him as Jacques's servant, is recognized as such and is right now actively working against him is a paradox. I would not put it past Jacques that Faust here is a servant against his will, but why rebel here and now?

If he is a bound servant that means that Jacques can punish him, so either Faust is certain he can get away with this (the possibility I'd consider likely with Jacques) or thinks that this is worth risking punishment over. If he's willing to do that, he'd be willing to lie to her for the same reason.

All plausible.

Also, to be honest, I am also suspicious of people that are doing you a big favor or being very nice to you, without disclosing why. Thanks to my emotionally abusive mother.

Oh, and one more thing:

It is entirely possible that he is saving her because the thought of Jacques having more children is sufficiently terrifying to Faust that he'll do anything to avoid that.

the theories are strong with this one

I mean faust not jaques lol

Cause he's lying and he isn't a real boy. Since this is not a Disney movie, this means something gruesome like bleeding.