Big Problem with Return at Macys

I purchased 2 Michael Kors tops at Macys on New Years Day. I tried to return one top today and when associate scanned receipt it showed already returned. Impossible! I still had the top in bag and it had not been out of my car. Looking at the ticket the proof of purchase sticker that Macys puts on all tags was missing off the Michael Kors ticket. My receipt showed that I paid cash but item still scanning as returned. I went to customer service and they pulled up my transation on 1/1/12 at 4:23 pm with my cash transaction and then showed same store associate returned item at 4:29pm same day and doing return on a gift card. The associate was a floater (I know the regular associates) and I think she did not put the proof of purchase sticker deliberately on my ticket so she could return the item and put it on gift card for herself (this would have been $110 plus tax). I don't think she planned on my returning the item. Security is going to pull surveillance tape and also track gift card to see if this is what happened. I don't know of any other possibility since I did not return item. Even if I had returned item 6 minutes after purchasing I would have insisted on cash back not a gift card. Any ideas?