So I can't exchange an item that still has tags and all

Im disgusted with the service that we got from Exact Lakeside Mall. My ex hubby bought a jean for my daughter at Exact and didn't give me the slip. I went to the store to ask them to change the jean size as it is small. The manager said that they needed a slip which I don't have. Then said that whoever purchased the jean must come to the store so that they can try look for the slip but if the jean was not purchased at their store then they can't help. The jean is still new still has a tag and all. So I dont understand why they couldn't help with getting a different size. Its not like I wanted a refund.