There's a manga I read a while ago, but I can't seem to find it anymore. Unfortunately, I don't really remember the plot well, but here's what I got. I think both of them are high school or uni students. Anyways, instead of admitting that they love each other, every time one of them wants sex, they pay each other, but it's a very little amount, like a 5 yen coin or something. This habit continues until they grow older and then they have a big jar filled with all of the coins and then buy a house of something like that together. If you can find it, I'll be forever grateful! <3

It is an extra chapter in Sekai wa Kimi de Mawatteru called Yasui Tamashii (Cheap Soul)

Yes, thank you so much!!! You are magical. <3