The scene will happen in a side quest for collect money. There will be a lot of side quest that wil lead to H-scenes. Your are not forced to accept these quests but if you do they will lead there. At the moment there are not many other ways to make money apart from side quests and hidden money around (but there are not many). Later there will be random encounters, monsters and salable items so you will be able to reach the money needed to buy the ring even without doing these quests.