Following up on the previous post: the Ketzos brought a proof from the fact that a shliach appointed to slaughter an animal for a ganav does not negate his shlichus if he does the shechita on Shabbos that if a shliach to bring a get coerces the woman to accept the get in violation of cheirem d’Rabeinu Gershom, his shlichus is also not negated. The Noda b’Yehudah, however, holds in this later case the shlichus is negated because ain shliach l’dvar aveira. We left standing the question of how the Noda b’Yehudah would distinguish between the case of get and the case the Ketzos uses as his proof.
Lots of nice comments on this one. Let’s start with a pragmatic chiluk: The shliach can certainly fulfill his task of shechita on a weekday as well as he could on Shabbos – the choice to commit chilul Shabbos, the act of chilul Shabbos, is completely independent of the job of tevicha. However, if the woman the shliach is assigned to deliver a get to refuses to accept it, there is no way for him to complete his shlichus except by coercing her to accept the get. The act of coerced nesina is inseparable from the act of nesina itself.
A more conceptual (this is suggested by Rav Amiel in his Midos l’Cheiker Halacha) answer hinges on a chiluk we discussed once before: there is a difference between the concept of shlichus and the concept of shlucho shel adam k’moso. A shliach sometimes is just a means to an end: the ganav wants meat, so he asks a friend who knows shechita to help him. Who does the shechita does not matter to the ganav or anyone else – all we care about is "where’s the beef". However, when it comes to delivery of a get, the shliach functions as more than just a messenger – the shliach must serve as a representative of the husband, because without the husband’s participation there can be no get! Delivery of a get requires shlucho shel adam k’moso – a representative form of shlichus. If the shliach serving as the husband’s representative does an aveira in the process of delivering the get, there is more of a reason to ascribe that aveira to the meshaleyach and invalidate the shlichus.
Just parenthetically, I enjoy R' Scheinberg's sefer "Mishmeres Chaim" because it is a treasure trove of these type questions. I began going through it many months ago on Friday nights/Shabbos with my son and have noticed that his ability to solve questions like this has increased dramatically -- I think it's not just that he knows more in general, but I think exposure to conceptual chilukim helps hone one's own ability to think along the same lines. One advantage to Rav Amiel's sefer over the Mishmeres Chaim is that it is organized around conceptual principles -- he sets out a type a chiluk or chakira and then illustrates it with example after example. The Mishmeres Chaim just groups questions according to the area of halacha they address, e.g. hil. eidus, hil. purim, hil. kidushin, etc. and you have to figure out chakiros and chilukim to apply at random. Perhaps a worthwhile project for someone with spare time and energy might be to re-organize the Mishmeres Chaim by principle, so you have the advantage of his wonderful questions along with a more organized presentation of the various types of logic used to derive the answers. Just an idea.