You will automatically begin this quest after speaking to Keltar at the end of the previous quest
Head over to the doctor and tell him it wasn’t Keltar, then return to Keltar for your reward.
Keltar is innocent? Hm... that's good to know. We almost incarcerated teh wrong person! But who then is the real killer?
Ha ha! The stupid old hag and her hammer nosed husband are dead. Now, I'm the sole heir! Excellent. Here's your pay for an honest job - or a not quite so honest job, ha ha!