This quest can be acquired from Johanka at the Sasau monastery. Speak with her about what happened with Matthew and Fritz. She will refer you to Brother Elias, who can be found in the same room treating the sick and wounded. Matthew and Fritz fought with Brother Elias, and now you must persuade him to let them off their debt. Use your persuasion skills or pay the debt yourself to settle the matter. You can now go and tell Fritz to complete the quest.