Thank you for your answer and for suggesting a solution.
Nevertheless you should at least be able to talk to Osyra about this. That she won't willingly part with her prisoner is clear but you should at least have the possibility to ask her about it. Right now if I want to talk to her, the game does not even enter dialogue mode but she just mutters something in the normal in-game view. Also there is no hint to kill Nedmar since she does not say anything about killing him.
I do see that this problem is easily solvable - as you described - but nevertheless this seems to be a story bug since it is completely unclear what the player is supposed to do and the game does not give him any hints.