The Extortion quest is available in Mayhew to Elves only.
To receive the quest, you must first successfully perform a one-man show in the theatre. Afterward, you'll be approached by a "sinister-looking" elf who tells you that he's looking for someone with acting skills to take the disguise of a merchants guild tax collector to extort and create trouble among the merchants. After agreeing, you'll be find an additional link in the Mayhew city menu to extort merchants.
Extorting merchants yield small amounts of gold which you can keep. The quest is perpetual and does not have an actual ending.