Ongaeshi (恩返し): returning a favor received, as in sewa ni natta hito ni ongaeshi suru, to repay someone who has looked after one.
Giri (義理): what one must do for others, or return to them, because of one’s position or as a moral duty in social life. An encyclopedia notes that always returning gifts given at weddings, funerals and other ceremonies is also called girigatai, being true to one’s obligations.
On o ada de kaesu (恩を仇で返す): to repay a favor with harm.