On "Ah, Get Me Next Time"

What's with going out with an acquaintance, them offering to pay, and then after you say Really Thanks, they say, "Ah, get me next time."

Wait, what? WHAT?

I get that this is usually acceptable, but when you're a broke vegetarian, the Ah-Get-Me-Next-Time is like the killer coming back to life (MINOR SCREAM SPOILER) after Sydney has shot him already! Good gravy--the stress.

A. What if the next time we go out, we go somewhere really nice? Is it still my turn to pay?
B. Frankly, it's about a 75% chance that I will spend less money than my dining companion--no matter what restaurant we go to because I don't eat meat, and I don't order alcohol.
C. What if we go out with other people? It seems silly to randomly announce, "Yeah, and I'm picking up his/hers too!" And then is the person thinking, "Why isn't Alice picking up my bill?"

When you have a system ("You do dinner, I do the movie") or something similar, that's okay. But, I'd really rather just hash it out or split the dang bill to begin with.