putting different prices on the same item

i was in this store on 04/08/2015 to buy an item that was on sale, since it was their first day of sale,,,,i was looking for medium size,,when i was there all the medium sizes didnt have a red sticker for sale, so i took the small size and medium,,,when i got to the till the item was 240.00,,,,but the red sticker was dispaying 125.00.....we argued in that till, it was me,supervisor and the cashier,,,they said i can only buy that item with a red sticker,,,,then i told them that as far as i know at woolworths if they are having a sale the price doesnt differ ,,,,so they refused to give me that item with 125.00 that was displayed on other items,,,,instead i had to take the smaller size which i was not looking for,,,why did the do this????