You could make it so the merchant only takes Diamonds instead of Cash, making Diamonds relevant throughout the game once you have all the girls.
"Cash? Get out of here, buddy. I'm all about those rare earth minerals! We're living in the future now, and all the future-proofing analysts say that the value of shiny metals everyone wants will never die!"
They could also sell temporary boosts and upgrades, like increasing the amount of XP a girl can get in a night so they can more easily catch-up with your higher earners.
Also, I don't like the "1 room, 1 girl forever" system, and you could make it so that you can assign girls into the rooms, or pairs to team up, or even multiples in one room to unlock special "mentoring" and "group" bonuses, where a more experienced girl can sacrifice some of her potential earnings to boost the experience gain of another, or you can put them into "VIP" clients that may force you to pay attention because they pay very well.