If I recall correctly, you will forfeit the quest if you don't free the imp properly. You simply won't be able to finish it. Asklepy will keep on asking where his slave is, amelie will keep on saying she hasn't found him yet.
I may be wrong though - I chose that in my very first playthrough and I think I lost the quest. Similar thing happens if you decide that you do want to be Teana's champion when you beat up arnold heninghem rather than refusing the title.
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