I'm afraid that this is indeed theft.
You returned an item that you didn't buy at your store, then bought the same item with your discount and points. Essentially you exchanged your discount and points for cash and walked out with the same item if I understand with the employee discount intact and the extra points due to event.
Not only would that be considered theft, but also fraud because of the returning an item bought at a different store. If that other store was another Shopper's then perhaps it's defensible, but if either of the locations is a franchise then they are separate entities and your store suffered the loss. Not only that, but just because your credit card was used for the purchase it doesn't mean that you made the purchase. And if in fact you did make that purchase, one has to conclude that the only reason you bought the item at another store (if a Shoppers) to return at your store was to facilitate the scheme of repurchase with your employee discount and points during an event.
Your defense is that the points program for employees and return policy for employees wasn't made clear to you. However, even if that's the case it's easy to see how your actions enriched yourself at the expense of the employer so even that defense might not be sufficient to give you the benefit of the doubt. If you're motivated to act, ask for a meeting with your manager to explain your side of the story more fully and see if they'll take you back. If they won't, then a letter from a lawyer threatening a wrongful dismissal suit unless they offer some severance and call it a "layoff" is still better than termination for cause.
So I'm afraid that the employer is most likely in the right here. I'm sorry if that's not what you wanted to hear. Is there anything else to discuss? If not, and if I've answered you fully, I'd appreciate a positive service rating please.