I've bought a fridge on account with Ok furniture for R2600 and paid for about 4 months and at that stage i have requested a settlement amount of which they said if i want to settle i should give R3900

I've bought a fridge on account with Ok furniture for R2600 and paid for about 4 months and at that stage i have requested a settlement amount of which they said if i want to settle i should give R3900. How is this possible ? And i told them that im going to return the item because it was not discussed when i purchased this item at the store for the said amount, so i did return it only to find out they reported me like blacklisting me and it shows negatively on my credit score. What can i do about this? How is it possible that the amount can now be more if you want to settle earlier and it was not told to me at first? What is to be done by me in a case like this? I feel very bad about this customer service.