Hi Fi corp and JDGroup dont know how to reimburse interest on account beware dont get caught

I bought some equipment at Hi Fi Corporation on account. Two months a juicer broke from (Mellerware) i have used it like 3 times when the blade stopped spinning all parts are plastic. I took it in for repairs after about 21 days i was contacted and told that it would be credited back to my account as i bought another one much better and better qaulity from homechoice a russel hobbs. So when i went to do the credit back to my account i was surprised to the fact that Hi fi management didnt know how to do a full credit. When you buy an item you get charged an sale amountR899 and then interest fir the duration of the contract amount. Now this interest which i was charged no one not even Head office can assist me i have emails from them. My interest was almost R10000 on equipment if R17000 so now Hi fi wants to give me the R899 as credit but the interest i still need to pay why ? As i dont have the item. What i do find strange is that how many people get caught by HI FI like this have you ever returned an item and was just given the sell amount. What about the interest that was included in to the full amount of your sale. 10 20 customers adds up to a big amount. So therefore ive put them to the sword by not paying my acc this month what else can i do as this has been dragged on by HiFi Corp. and there is absolutely no contact between JDGroup and customers nor from HI FI corp.