After your first time helping Arniel you can return to him and ask him if he needs anything else done. This time he’ll ask you to go speak to Enthir for him, who is backing out on a trade.

Go speak to Enthir who will CORRECT you and tell you Arniel never PAID for the item… so if we want it we’re going to have to go get a particular staff for Enthir. The staff is (I suspect) put in a random location in Skyrim for you to go grab. For me, I had to travel to Cragwallow Slope (a cave just southeast of Windhelm).

So, fast-travel to the nearest place you can get to and go through whatever dungeon they want you to to find this staff. Once you obtain the staff (and kill whoever DOES have it), return to Enthir and trade it to him for a Warped Soul Gem , which is what Arniel wanted. Now simply go back to Arniel and give him the soul gem to end this quest. WITHOUT a reward, I might add…