Unprofessional behavior

I bought a pair of puma sneakers for my son , when I got home my son fitted them and they were small , while he was fitting the shoes became a little bit dirty. So I went to change size but were I bought them they didn't have the size, I then went to Studio 88 in Pretoria Bloed Mall when I met this lady whose name is Tsholo / Kholo the lady refused to exchange she said the shoes were damaged and it's a lose to her store so it's my lose or I should take them back to where I bought them and tell them to give me clean shoes cause she won't exchange them