Champion/Skin Sales

So I noticed that starting 09/05 Rocket Girl Tristana is going to be on sale, though I just bought that skin last week. Is there a way that I can't get the difference, like refund the purchase and re-buy the skin at the lower price, or am I out of look and just wait and hope things go on sale. I'm sure this has been answered before but please and thank you for your response.