From my understanding I need to:
1. Defeat Ceres for the iceflake
2. Lose/give up in a fight while Galiene is with me so that Galiene gets possessed.
3. Come back to the ship, say I'm okay with letting her posess Galiene for now
4. Let in-game days pass by sleeping
5. Return to the ship and get an option to hire a possessed Galiene for 100 coins
I've done all of that and did not get an option to hire Galiene for 100 coins.