Harvey Norman complaint: Can’t refund can’t get store credit

I needed a cable that can connect from my computer to my desk top so I can work with 2 monitors. I told him I wanted those 2 monitors are able to show me different thing. He straight away gave me this item. I didn’t know anything about it. I thought he was correct. So when I got home, I was excited and open the package roughly so damage a bit of the box . But when I tried it didn’t work at all. Nothing shown on the second monitor . Then at work, I ask the IT guy he told me o don’t need that expensive thing. I just need HDMI cable about $10. What I need to do is change the system to be able to work on 2 monitors with different view. So I went back to Harvey Norman and told them I can’t use it and I wanted refund. They rejected, they said, the box is damaged and I changed my mind. I told them I couldn’t use it! That’s not what I need. I didn’t chose it. Their staff chose it for me. I believed him coz I thought he is the expert in that area. I am furious . It’s expensive, I can’t use it and they rejected to refund me or even to give me a store credit. Please help!