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