Not happy at all

I bought a jacket for my daughter and it was on promotion or sale, it became small for her. My sister went there to exchange the size and the lady by the teller said to her that she needed to add more money to exchange only the size. Where on earth was that even possible because their receipt says they'll exchange without any charge or additional money? I'm open to be corrected but this is far from providing a good customer service let alone customer sertisfaction. Please help maybe my facts are not correct.