Expiry of special

I wanted to upgrade to a Samsung Note 4 but they were out of stock. They managed to get me a Note 4 at another branch but it only arrived at their branch on Monday. I went to the store to collect my new phone yesterday only to be informed that the special expired on the 7th of February and if I wanted to upgrade now, I must now pay more. I feel this is very unfair as I would have completed and signed the contract before the 7th should I have known that this would be the case.