Overcharged

I was in the River square store yesterday and bought a box of oranges. The box had been marked down to R9 because it was a couple of days beyond its expiration date. However, it rung at full price at the till. I noticed it on my slip just 2 minutes after I exited the store and when I wanted to go back in to query it, the security lady told me that everything is already off and that I have to come back another day. I explained to her that I don't live in the vicinity and it's quite a drive, but she was adamant that she cannot let me in. So I tried to go to a closer store today and they told me they could not help because it would be a loss for them.