Square has an offline mode that does this. Cards are queued up and authed when the device goes back online. The merchant accepts liability for declines, but it'

Square has an offline mode that does this. Cards are queued up and authed when the device goes back online. The merchant accepts liability for declines, but it's a good deal in exchange for being able to run their business in remote locations or during some kind of outage. It has to be explicitly enabled so you can't just trigger it by knocking out WiFi.