Refund problem and poor staff knowledge.

I bought a Fitbit versa 3 smar****ch at Incredible Connection in Greenstone Shopping Centre in November 2020. The watch worked for 2 weeks and after the Fitbit app update on my Huawei P40 Lite 5G, my watch stopped connecting to my phone, receiving notifications and switched on and off by itself. The result thereof was that the watch wasn't compatible with my phone and wouldn't work after these updates. I took it back and requested a refund after they had also told me that it's incompatible with my phone. They then said that they will be deducting 15% as the product had been opened, by one of the staff members in the store, which I find to be unfair and unreasonable. The staff aren't trained well enough to know that it wouldn't be compatible with my phone and therefore couldn't tell that to me. Even though the watch was still in it's original condition and there was nothing wrong with it, except for the incompatibility, they still went along and took the 15% for something that wasn't my fault or as a result of my negligence, and after telling them that they aren't supposed to be deducting the 15%.