Let's analyze the situation: you accept his "offering", and get cursed. To prevent any further problem he will then kill you, which no longer can pose a threat, and do the same, or worse, to your daughter. Bad ending.
OR
You kill the asshole, right there, right NOW. No thinking. No warning.
Then you kindly ask Alan to release your daughter: since he is supposedly keeping your daughter as a hostage he is either near, or he is monitoring the situation with some magic means, what it is important is that he is listening. He also doesn't have any interest in keeping the girl hostage any more, since he has seen that having her doesn't prevent your death. Also the mage that should remove the curse is dead anyway, so the original plan is already in the shitter. His choice at this point would be either give back the girl, and spend the rest of his life regretting his choices, or not doing that, and be a cursed cripple on the run from an extremely pissed top-grade adventurer, spending whatever is left of his life SORELY regretting his choices. In any case you are likely to get your daughter back sooner rather than later, since the mage is clearly the brain of the operation (by the means of not being the one cursed). Happy ending?
In any case get a refund for the crappy bird, since it is a waste of oxygen.
What instead would happen is that he will agree to get cursed, we will get another rant from the asshole, he will try to kill the idiot only to get stopped by some of the finest asspulling (the girl using her magic power is at the top of the list, but I am not excluding Alan regretting his life choice and trying to redeem himself). Douglas will definitely not punish the mage, who will either get away scoff-free, blown up by his own petard or killed by some random passer-by (the barbarian, mister Tolkien's butler or the innkeeper, just to make a few names). Alan will definitely get a redeeming arc and the protagonist will get rid of the hex for good.