Bad service

On the 04 June 2021 I purchased an office london dress online using my Truworths account.The price was R240.I collected it on the 12 June 2021 from the collection store where it was delivered. On the 15 June I went to Office London store on 387 West street, Durban in order to exchange for a bigger size. I was assisted by Patience who told me I will have to pay R320 for an exchange since the item is no longer on sale.I tried explaining that I'm only exchanging for a bigger size but she said it cannot be done. I informed her that I only had the dress for 2 days and was purchased online but she said it's the store's return policy to return items within the sale period. I was baffled since it was delivered probably after the sale but I'm expected to return it within the sale period.