Vishal Mega Mart — changing of wrong size ladies panty

I have purchased 3 sets of ladies panty on 30th april and after reaching home i find one pair size less than other ones. Tag was xxl like other ones but it was of small size. I tried to next day to change it with correct size pieces but store manager denied to change it. This small size pair is of no use for me and is as it was purchased with tags. Due to wrong size tag. Please arrange to change it. This is only measured with other pieces not used.