Reliance Trends — Not allowing me to exchange clothes which they have wrongly put in my cart while billing

Address:

Reliance trends Hubli.

While billing the concerned person has exchanged my pants with smaller size and I did not notice it while collecting them ad there were many cloths. When I go to exchange today they say it not possible. What do I do with the dress which doesn't fit me. And it was sthe mistake of the concerned staff who wrongly put my pants with other size.