mark up price

I bought a shoes on my account on de 3 may it was on sale for R110 den i wear them on monday only to find out that other shoe is damaged. I went to the store for exchanging then the shoes scan R140 same shoes same colour. The store manager by the name sindy told me dat dey r on mark up now i told her that the reason i bought them is because it was sale so is not like i am buying again she say no and cashier shouted at me and telling me dat she is crediting back the shoes is not her fault that I used of cheaper stuff. I leave them they force me to sign the slip i refuse cause i didn't buy anything so y i must sign.