Angry unhappy customer

I bought a shoe at Foshini Mountain Mill Mall in Worcester. I took the shoe back 2 weeks later because the 6 was n bit too big. The straps kept sliding down which made it impossible for me to walk in. I literally walk with the shoe from my flat to my car in the parking lot. When I took it back the 2nd in charge said she could not take it back because I wore the shoe. I explained to her what happened. She then spoke with her area manager Mr Botes which said that it was not a factory issue so they could not exchange the shoe. As a consumer I have the right to exchange within 30 days of purchase. I have been a client of Foshini for many years and I'm unhappy with what happened. They expect me to pay for a shoe that I will not be able to wear. I believe also that if it had to be a white lady they would instantly have exchanged the shoes for her