Incorrect price

Ibouggt a dress from Edgars for my daughter which was on mark down, i take smaller size, when i got to the till the lady scan and say its incorrectly marked down and i am not allowed to take a smaller size for the same price as they received stock differently the big dress is last years stock the smaller dress this year, what nonsense is that 2 identical dresses two different prices. They refused to give me the smaller dress. I will not leave it here.