Repairing a laptop

My mother bought a Lenovo laptop for me to ise at school with an account with Russells and she has been paying monthly, the laptop was faulty since I started using it but couldn't take it back sooner because my study's have already started. When I finally got a chance to take it to them for repairs or getting a new one they started telling me that it has water damage and stuff which is not true it was faulty from the beginning and they don't want to fix it and also want R 5000+ to fix it. This is very annoying because we are not even done paying for that laptop and if they don't want to fix it it means we are paying for a damaged product already