If the computer is down, no sale can happen.
http://www.cashandcredit.com/index.php?l=product_detail&p=31...
Some years ago, I went out for coffee with a friend. The coffee shop's card terminal was down for whatever reason. I could see an imprinter and a stack of the slips on a shelf behind the counter. I even offered to show the barista how to use them, but she...ah...declined.
I can bet $50 the total of 'carbon copy machine' in the whole country is the double digit number, only because some biz older than 20 y/o are still in the biz and they have it somewhere in some closet.
Edit: ah, yes, imprinter
The receipts are issued by machines that can sync with the government services frequently, but it's not synchronous. All transactions are written in a special "flash" memory and synced via the included data sim card each 5-10 minutes.
Buffering up to 24 hours is possible.
The EU has been going down the rabbit hole of administrative madness for years now but that should not stop anyone from facing emergencies.