This is the summary of a short story.
There's a man who's some kind of freedom fighter on an alien planet. He takes pity on a slave girl and releases her from bondage. He almost makes it to safety with her, when her "owners" show up and inform him that he has made a wasted effort as all their slaves are poisoned, and if they don't get the antidote daily they will die. It has been too many days to save her.
I read it several years ago and I can't pin it down to a particular decade. I know it was pre-2000.