Mr Price Factory fault

I bought shoes at Mr Price. when I got home the left won't fit but the right fits and they are both written size 4. I then took it back and they said i should take something else because i lost the slip. i then took something else but when i got to the till the shoes were on mark down, the told me i had to top up or take something worth the mark down price. i refused and explained to them that in the first place i was just avoiding a conflict and trying to be polite. it it not my fault that the shoes are labeled incorrectly, it is their factory fault and if the shoes were not faulty, i wouldnt be returning them coz i was going to wear them the same day. all tge agents i spoke were not helpful and they still not willing to accept their shoes back. i feel insulted coz i think Mr Price is telling me in my face that i am disabled. thats why they insist i remain with shoes of different sizes. i just want my money back because i have no confidence in their products anymore.