Bad excuses due to human error on product pricing

I bought a GTX 960 online at First Shop on the 23rd of December 2015. The product was advertised at a very good price. Now today on the 5th of January 2016 they sent me an email and telling me that they cannot give me the product at the advertised price and will reimburse me with an amount of R988.06. The amount that was deducted of my credit card was R1078.06. The problem is that First Shop has a different story with different clients every time. My colleague at work bought the same product at the same amount and the First Shop representative told him that there isn't any stock which is a total lie. According to me if you advertise a product for a certain price and you deduct the amount of my account for the product to complete the sale then the product must be delivered to me. My First Shop reference is FS919613.