Sure you can
You can always go back and tell them that you want to return the item. If it's open, they will charge you a restocking fee. If not, that's much easier.
Anyhow, if you already opened it and want the ipod Touch free, I'd suggest you to go back to the store with all the items appropriately packaged (AND RECEIPT), and I'd explain that he didn't know about the promotion and that he would like to participate on it.
9 out of 10 times they will help you out, especially because they want to get rid of the old ipods because the new ones will be released on Wednesday (09/01).
Don't even think about waiting until after Wednesday. The new ones never apply to that promotion. Otherwise everybody would wait to buy their computers during the last week of the deal.
Let me know what happened.