This is a short story or novella, not a whole novel.
So there is this woman on a generation ship, in her luxurious quarters. Her boyfriend has just travelled somewhere -- the ship is huge, so travelling to different parts can take days or weeks. While he's gone, some man knocks on her door looking for him. The boyfriend owes him money or something? The woman thinks she's being scammed and she won't pay.
Then somehow it comes out that her boyfriend is one of the caste who works on the outside of the ship. They become disfigured (from radiation, I think), so no one wants to do it, and no one with money ends up there. The woman objects -- the boyfriend doesn't look disfigured. Then the man tells her there's an operation to fix it, and that might even be why he's owed money.
The part I can't quite remember is this: somehow the woman agrees to go and work on the outside of the ship instead, like in response to blackmail. And in order to do it, she has to get an operation so she looks like the other caste, for which she pays.
She does it, and goes there, and works with people on the outside (including another woman). I don't remember the details but eventually she wakes up back in her old apartment and is initially panicky -- now that she's disfigured she won't fit in! She looks in a mirror and realizes there never was an operation and it was all a ploy to scam money. Her boyfriend shows up and she feels sympathy (the part about him being one of the other caste was true), and vows never to tell him that she knows. The end.
I read this probably in the mid-1990s. I feel like it was in some "best of" collection or other.
