Haruka's debt is 100 Ryo and you pay it back in 5 installments of 20 Ryo. If you get a random letter when you're at your house in another life (you get one when you leave the house, to go into the garden area) it means you can pay back one of the 20 Ryo installments (though you have to read the letter first - from the start screen). When you have 20 Ryo, go to the shrine in Fushimi and talk to the man there and select the top option to pay him. After paying him 20 Ryo for the first time, he gives you the Genkimusume Kimono, which you can give to Haruka (when you return to the house with it, Ryoma automatically gives it to her). Now when you select the little drawers by the cupboard (the one you examine to sell items), you can select the middle option to change Haruka's outfit (you can get more Kimono from the Shinto Preist's shop).