Disappointed

So I’ve lost my receipt and have no bank statements. I’ve got a pair of shoes as an gift but I’m unable to exchange it for a smaller size. I called the stores and they refused to exchange without proof. The item is brand new gifted to me and bought from the Birkenstock store and they refused to exchange. I’ve been trying to contact the head office but there’s unfortunately no answer from them. So now I’m sitting with a pair of shoes worth 1600 that I cannot wear.