exchange of my son's sandals

On the 20/10/15 I went to pep in campus square to buy my toddler two pairs of sandals I took size7 and when I got home the shoes was to small on the 21/10/15 I did what any mother would have done and took the shoes back for a bigger size please don't get me wrong this is not my first time buying small shoes for my child it happen a lot but today I felt like it was the first time I took shoes back I was told that they cant exchange the shoes because its dirty I couldn't see its dirty because if it was dirty I wouldn't have brought it back in the first place and then they said I also didn't have the price tag so they cant help me at all I felt so embarresed because the manager went on telling me that maybe I fitted the shoes on for my son with dirty feet WOW the people in the que even started laughing never ever was I treated like this and I will not accept this the same shoes they claim to be so dirty was exchanged at eldoradopark pep stores and without ant hassles and the same dirty shoes I took a picture in the shop as proof because I know they will deny me why didn't I have any issues changing the shoes at pep in eldoradopark the manager there told me not to leave this as is