Bad customer service, should I be billed for truworths error

Today I had a bad experience at Truworths in Blue Route, Cape Town, I went into the store to purchase an essential item, on arrival I was ecstatic to see that what I needed was marked down by 20% only to get to the till to be advised that the promo has ended and the product needed to be purchased at the original price. I asked to speak to the manager Linzay Booysen who rudely advised that there's nothing she could do and I could pay normal price or leave it. She really was not helpful or empathetic in any way. She told me it was human error and she couldn't assist me. How am I supposed to be paying for a human error on their side. I should be paying what the item is marked at. She told me pay for it or leave the Item. I need someone to assist me or get back to me, because if this is the serve offered by Truworths I am appalled. She said what the wanted and walked away and she was not even willing to give me her name and surname, she said she couldn't but I later saw a display board with her details.

Call me or email me I will forward you the proof as well as the mark down ticket on my garment.