price adverised lower than price I had to pay

I saw a Defy Washing Machine advertised on posters outside the windows in Cape town with a Defy toploader metalic finish 8kg washing machine selling at R2589 and I went running to the BlueDowns store but upon swiping my card I discovered that I was paying R2789 for the machine and I was told that that's because the price on the windows outside is wrong. I do not understand how that could ever be possible because on the poster outside it said that the prices are valid until 27July and I was buying the item on the 24th. They know exactly what the CPA says about this issue and I should be compensated for the difference in the amounts and therefore I WANT MY MONEY. THEIR ADVERTISING IS A LIE AND MISLEADING!!!