An Unpaid Debt is a Side Quest in The Witcher 3. This quest is only available if you finished the Stranger in a Strange Land side quest and used Simun Brambling's help to escape from prison.

To activate this quest, go to Kaer Trolde in Skellige and look for Simun in the New Port Inn. Simon will ask you to meet him in Blandare to help him with some business. You can refuse to help, but the quest will be over.

If you accept, meet him at the hut between Rannvaig and Blandare. Simun will ask you to kill a man named Volund.

You have a choice to make:

1. Repay your dept and kill Volund for Simun.
2. Refuse to kill Volund and face Simun instead. You have to kill him.

Both options end with the death of a man who has done Geralt no wrong. It's up to you to decide whom you trust.