Why do i have to pay for store mistake?

<p>Last week I saw a special at Footgear Table View for R300 Puma Takkies/Snikkers. I went there on Wednsday the 9th Nov 2016. I took 2 X R300 snikkers as per the price tags but when I get the to the cashiers , prices have suddenly change. The other one was R400 and there other one was still 300. When I told the cashier that the price is 300 he said it was the mistake, a mistake of 100 is too much. so I contacted the manager and he apologised and said he will not give me for 300 but will give me for 400. this is so unfair, why am I paying for store mistakes?</p>