Well either
the witch hunter is doing their thing, hunting witches, when suddenly they are afflicted by a really awful hex. Normal medicine can do nothing for it, and the only one who could help is a magic user, but of course no one is really keen on helping. One magic user steps up and says they will help, but for a price- the hunter will receive treatment only if they go after a very specific magic user, which turns out to be the one who cursed them anyway so it's a win/win really. However they are not allowed to go after any other magic users, if they do the magic user will cease giving aide and the hunter will die an agonizing death. The magic user only heals enough to keep the hunter going for a day at the time and promises to heal the hunter completely once the task is done, however they don'really have an intention to keep their promise at the beginning of the quest.
or
The magic user is the leader of a small group of magic users on the run. The witch hunter is hired to go after them, and chases the leader relentlessly. However, they know each other only the magic user is aware of this because generally they hide their identity when with their group. Now the magic user must convince their friend to cease their hunt and have a change of heart, or their relationship with one another wil be destroyed.