Refusal to credit back or exchange the shoe that won't fit

I bought a pair of shoes for my husband. The sales person gave me a size 6 shoe inside a size nine box, unfortunately we list the box and only realized later that the shoe didn't fit, however we still have the receipt. Now the store refuses to take back the shoe or exchange it for us because we lost the box. We couldn't have bought the shoe at any any other store accept spitz, I thought they know their products. I'm so disappointed at the level of their comprehension and processing of information even after they seen cctv of when the purchase was made they still refuse to help.

Do we really have to involve lawyers in every little dispute that can be easily resolved if we apply our minds. Really disappointed