Did Mami stop their car just to tell them she quit? And what stops them from fleeing with the money?
page 29: "They might already be looking for us. If we're seen by a policeman, it's over."
And then on page 31, who shows up but a policeman? No doubt he'll investigate their car, suspiciously parked in the middle of the park. They certainly can't drive it out of there!
So I think the money will end up being returned to the rightful owner, even if Mami had no idea what was going on the whole time.