The math here depends entirely on what the "certain rights" Lucius has over Harry are. Were the debt a purely financial issue, saving Hermione would be a no-brainer. Did those rights allow Lucius to realistically cripple Harry's efforts to fix the world, not saving Hermione would be a no-brainer.

We still don't know, although the fact that Dumbledore ultimately went along with it suggests that it's closer to the former than the latter.