Terrible service at iStore Canal Walk - Rude manager

On the 15th Nov 2022 I purchased a charging pad at the store in Canal Walk. I realised it was too slow and decided to return it as it was not serving its purpose. I returned it on the 17th Nov 2022 and the manager was very rude and told me they would deduct 20% as I had opened it. How was I suppose to know that I will not be happy with the performance without me opening it? On top of that, the assistant that was helping me did not inform me that should I open it, I will be charged 20%. i am very disappointed as I own a lot of Apple products and you have definitely lost me as a customer