The choice is not between lying and telling the truth (even in a roundabout way). You have many other options at your disposal.
You can, for instance, refuse to speak.
You can attack and kill the Nazis, who would, after all, unjustly attack and kill the Jew you’re hiding if they had their way.
You can, when they ask you “Is Herr Schwartzmann here?”, respond, “Yes. Wait right here.” Then close the door, grab Herr Schwartzmann, and run out the back. You didn’t lie after all, though you did deceive.
You may say that none of these things are feasible, that they would carry the risk of ruining your own life. But then you did the Jew an injustice by agreeing to shelter him only to hand him over the moment it becomes an inconvenience to you. If you are not prepared to protect him, you should never have agreed to shelter him in the first place.
As for the difference between lying and deception, it all goes back to the reason why lying is morally prohibited: it subverts the natural end of our communicative faculty, which is to relay truths. Deception is moral where lying isn’t because deception offers the opportunity for the truth to be discerned (e.g., when you close the door in the Nazis’ faces, they may discern that you might be trying to escape) where as lying doesn’t.
All that said, lying in a situation like that is, as you said, I think a venial sin. And while venial sins nevertheless ought to be avoided, I really doubt you’d find your soul imperilled by telling a lie under those circumstances.
