Cooling off period

Good day My father in law took out an upgrade with MTN on the 01 June 2020 and when he got home the phone was not friendly user and ocould not use the phone and his old catt use it the phone its Huwei P30lite and he wants and LG Q60nd took it back to the store 07 June 2020 under cooling off period and was advised they won't take the phonestly if there is nothing wrong with the phone.Can someone please explain how this works as I also work for a service provider a day I know the phone can be returned if it not fit for the purpose if it's still within 7days