unresloved query with edgars store

Can you please help ,have purchased a cell phone 2018 December in one of Edgars stores for cash R3000 as normal they phone was checked n was was told if not working within 7 days to bring back and the phone has a warranty of a year. Same time passed by a had a problem with phone not reading the sim card so i sent it to the store and was advised to send it for check up, obviously i did n at that time we checked condition of the phone and that was put at submission documentation.of which both the store keeps and a get to keep a copy.Just after few months went back to get the phone was told , the phone had bigger damage and therefore i have to pay for repairs expenses. Ofwhich cost R3500 of which did not make sense to me as i know the phone had a faulty with sim reading not what the are detailing to me .My query is what advice can u give as the phone is still with them and the group might be cloing down.