In Israeli Dati social media circles there's been a little bit of a buzz recently1 over a segula apparently first introduced by Rabbi Grossman during the Second Lebanon War (see here from 0:52). The segula is having a soldier take a shekel or another amount of money with the promise of giving it to tzedakkah after the war, and to keep it on his person when he goes to battle. That makes him into a shliach mitzvah and "shluchei mitzvah aren't harmed".
The buzz comes from the alarm of people who wonder why going out to a milchemet mitzvah, protecting Klal Yisrael from their enemies, isn't "mitzvah enough".
I was therefore wondering two things:
Is there a source for what Rabbi Grossman suggested? What I mean is that besides "shluchei mitzvah aren't harmed", is there some old custom to give someone tzedakkah money when he goes to war or to a dangerous place?
What is the rationale for making someone a shliach mitzvah for one mitzvah as protection while he is already a shliach for another mitzvah?
1 For future reference, this was amidst the 2023 Israeli-Hamas War. I like the title "Mashiv Ha'ruach War" used by some.