Kitty Catering - Galdin Quay
Quest Giver: Mulbrydale Notice Board
Other Requirements: N/A
When you visit Oxenfurt's market district, if Geralt has over 35,000 Crowns in his inventory he'll be approached by a tax collector. He will ask you a few questions about how you came into all of your money - if you answer yes to two of the questions then you'll be given a summons to pay back taxes. You'll need to take this to Novigrad where you'll pay the man at the bank.
During your dialogue with the tax collector he will reveal one of the best ways of making money in The Witcher 3. You can purchase sea shells and then dismantle them into Pearls which you can sell back to vendors for a profit, this is question 2 in the tax collector's dialogue.