I have some questions during all thoses years that now it's enough, I still don't get them and I need answers.

In
Dirty Harry, the scene where Harry and his partner Chico are going to do the surveillance on the rooftop.
Once there, Chico asks Harry:
How about the priest?
He knows he's being set up for bait?
Harry
Yeah, he wanted to.
We told him we had a volunteer
willing to take his place.
Who?
Harry
Never guess.
Chico laughs and says
Yeah, I know.
Welcome to Homicide.
He got what Harry meant and I don't.
Who was the volunteer?

Again in
The Enforcer, at Mustapha's office, as Harry leaves Mustapha says:
Callahan, you go out and put your ass on the line for a bunch of dudes who'd no sooner let you in the front door than they would me.
Harry
I'm not doin' it for them.
Mustapha
Who then?
Harry
You wouldn't believe me if I told you
Mustapha laughs and again he got what Harry implied and I don't.
Please, what did Harry mean in those scenes?? Who he was referring to??