Sylvester Amello a Novigradian merchant with some problems of imp kind, or so he think and so he hire Geralt of Rivia witcher to investigate this issue.
While Geralt solving this he will realise that it is in fact doppler that he is chaseing.
If you let the doppler go, Sylvester only offers you half the reward. No trophy. No full payment. After the reward, the quest concludes.