Mistakes certainly do happen and details are sometimes (though infrequently) omitted from receipts. Nonetheless, if all other aspects of the receipt are correct, then I doubt the seller’s insistence on the receipt being incorrect is justified.
Frankly, you should have the bag authenticated. Any credible authenticator will be able to tell you what the leather is, as Swift and Box Calf look very different. You could also post a close, clear picture of the item in the Hermès Identification thread, and I or another member should be able to tell you what the leather is.