Bed complain

I bought a bed at sleepmasters 2 months ago, when I got to the store I told the sales person which brand and size of the bed I wanted but I was convinced to buy another size because they're differences is just the palm of hand and the price is the same, I paid R6300 and I was told the size that I wanted was going to take about 7 days to receive after paying but if I buy the one that I was convinced to buy I would get it same day, I got the bed from the store same day but after a month I noticed that the bed is not really comfortable and iam suffering from back pain, I reported the matter to the store but the manager told me that they won't pay me back I'd exchange the mattress I told him I want to upgrade and he refused saying I slept on the bed, the warehouse house called me for mattress exchange but I told them I needed to change or upgrade the bed since I am not happy ,they did not give me the free gift (speaker) manager with a negative attitude said I was gonna get the free gift only if it was from the warehouse. It's been over 2 months now complaining but no help.