Unhappy client

Hi, I'm very much disappointed as I went to do exchange on the item I bought there, I lost my slip but the item has a price tag so they told me I should pay extra money as the item is no longer that price it's marked down to R60 from R79,99... Now my child won't have a trouser to wear on Xmas day. And I really don't know what to do with the trouser as I was about to buy the one that is R99.99 which would fit on my child.