Is it permissible to refuse to play movies for the elderly on the grounds of their impermissibility (haram), and does this entail sin, or the obligation to return part of the salary, or an expiation?

No one has the right to disobey Allah Almighty. Preventing what is unlawful is a form of assistance towards good and advice for people. As long as the aforementioned films are unlawful to watch, the questioner has done well by refraining from showing them. He should seek Allah Almighty’s forgiveness for the times he played films that are forbidden.