I have done some testing on this and it does not appear to be level based, at least on the level when killing the zombie.
The reason for my testing was that I ran across this thread when trying to figure out why the zombie was not reacting to me when speaking to him. What I discovered was that if you blow up the alchemist's gear and do not talk to the alchemist before going back to the witch, it bugs the quest.
So, I was level 8 and wanting to get an INT ring for my mage, but not able to get the quest from the alchemist. I reloaded from a level 5 save, prior to doing the witch quest and have killed the zombie (reloading each time to try again at the next level) at levels 5-8 and he always gives me the copper ring, which I do not want.
So, it is perhaps based on your level when receiving the quest, but I am not starting back at level 5 again. This is my first play through, on impossible, and I do not have that much free time.
If anyone else has information on how you can change rewards, please post.
Best,
Arc