i brought varus the last 7 days ago and its in promo now

https://support.leagueoflegends.com/home

Open a support ticket, they will refund you.

I should add to Tyrion's post that as long as you buy content two weeks max. prior to its sale, the difference will be refunded to you.

Contcat support.

tough, happens really. should buy champs with IP, thats what i do, otherwise its a waste imo, then buy skins with RP.

you can buy champs with IP and RP but only skins with RP. remember that.

I always wait to buy champs on sale for this exact reason. Contact support.