In the Star Trek: Voyager episode Phage, a Vidiian steals Neelix's lungs to replace his own which have been affected by disease.
Janeway is not willing to simply take back the lungs by force as she doesn't feel right killing one creature to save another.
By the end of the episode, it's determined that while none of the crew have lungs that are compatible with Neelix, the Vidiians have advanced medical technology which will allow one of them to be transplanted anyway.
Now comes the moment of having to choose a donor, someone to give up a lung so that Neelix can live, and Kes heroically steps up.
Why did they not just take a lung back from the one who stole them in the first place? He even seemed genuinely concerned for Neelix and so would not have objected himself, it would have presumably been the best possible match (being Neelix's own lung), it would have gotten Janeway out of her strange moral quandary and it wouldn't have discommoded one of her crew!