Bad Service at Woolworths Carnival Mall

I received a dress as a gift from my daughter in December, it has not been worn, it’s still has a tag on it, we took the dress back to Carnival Mall Woolworths to exchange since the dress does not fit anymore we were told by the Manager that can’t happen because. • One the price on the system has changed • And we don’t have a receipt for the dress I have never experienced this as other Woolworths now I want to know why is this an issue as we got the dress as a gift and it still has the original tag on it why .