Oleg Cassini 5438 Size 6

My daughter is getting married in April and we purchased a size 6 Oleg Cassini 5438 wedding dress. She now needs a size 8 and because I cannot find the receipt Davids bridal will not exchange it. Never been worn of course and the tags are still on the dress, paid $900 but will sell it for $700, therefore someone will be getting a deal! I will then have to buy her a new dress size 8 and will eat the cost.