If you have the first summoning quest and read the summoning book in the library to find out which ingredients you need, you should be able to ask Nixfix about the kohar root. Nixfix says he has some, but it is for his flooge frog, then mentions that flooge frogs like quetzalcellona mushrooms even more than kohar roots. The next time you talk to Nixfix with the mushroom, you should be able to say 'I have the mushroom' to initiate the trade.
