Soroban advises you to pay the Ruby Tithe so you can avoid conflict with the Confederacy as you cross the Ruby Sea.
This quest is available if you have completed the quest Once More, to the Ruby Sea.
Starting the Quest
Talk to Soroban in the Ruby Sea to begin the quest. If you aren't currently in the Ruby Sea, it can be reached by going west from the Shiokaze Hostelry in Kugane.
Go west to the quest icon on the map. Talk to Lyse to proceed.
Speak with Soroban
Go west to where the orange arrow is pointing in the minimap. Talk to Soroban to proceed.
Speak with Tansui
Go north to the stairs and go up them. At the top, go southwest to the quest icon on the map. Talk to Tansui to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Tansui to accept the next quest: Boys with Boats. There will be a cutscene.