Wrong promotion price not being honored.

I went to the Spar De Tyger store on Heritage day to buy a few things, and to my surprise there was a special advertised on the shelf that Douwe Egberts 400g is going for R269.99, but when I got to the till it scanned R319.99, the cashoer called the manager and even went to the shelf and saw the price being advertised, the manager on duty then said it has not started yet and he cannot honor that and if I want that price to come back on Friday, as this is a merchandiser mistake. I don't care who's mistake it is I want it at the advertised price thqt was on the ahelf in store.