Not 'Split Decision'.
Another hint: the person Mac says it to, has known Jack Dalton.

MURDERER'S SKY?

Legend of the Holy Rose.

Sorry Cirubit. Mac'God, the floor is all yours.

'Used to be.'

Hint: It's about a past event.

I can picture him saying it, the way he says it...its on the tip of my tongue!!

Another hint: Something was there before.

That hard?

Major hint: The place got torched up.

HELLFIRE?

Too obvius?

Wrong fire.

Ceasefire!!!!!!!!

Wait, no, I think that is also wrong.

Wrong too.

Hi guys, I'm back! Missed me?

Pete's cabin was burnt down by Murdoc. In the last episode of season 4, 'Unfinished Business', when Mac and Jack Dalton are looking for Pete, Jack refers to Pete's cabin being near. Mac's reaction is 'Used to be'.

I was wondering what kept you? I knew, you knew.

You're on.

Well, I had to be somewhere else, as usual. Preparing a trek through the South Algerian desert at the end of this year. And hoping it will come through – the region can be pretty unstable.

Let's see if we can find a nice new quote.

How about this one:
'Whaddaya say we talk this face to face, huh?'

Face to face, huh? There's only one episode that comes to mind and it's not that one. That wasn't about talking.

Ah, you mean 'Humanity'! 'You're gonna have to shoot me face to face', right?

Yeah. But that's not it.

No. And that one would be too easy for you, because you seem to know this episode by heart.

hmm

There But For The Grace?

Excellent, Mac'God! It's what Mac says to Doc right after Doc has caught Mac snooping around in his corner under the bridge.

'What's in it for you?'

At first this looked like a hard one. Then I thought: this is a typical question Mac would ask Jack. And going over several Mac&Jac episodes I am pretty sure it was in 'Mask of the Wolf'.

It's not a Mac&Jack episode.

Now I have the same problem as I had with Cirubit's quote a while ago. I am positive Mac uses this line in 'Mask of the Wolf'. Apparently he also says it in another episode.
So, to avoid confusion, let's use quotes that are unique, and I mean: used only once.
Okay?

If the quotes aren't unique, maybe we can try to add a hint immediately so it might eliminate too many possibilities. Like names though it might be a dead give away than.

Hint: this was said over lunch, they were after the same person for their own reasons but they worked together after an awkward meeting.

Actually, the first episode that came to my mind was 'The Black Corsage', with Mac and Frank Colton talking over lunch. However, the quote you want isn't used there.