Which countries exactly? I've traveled in UK and France and in both countries when the online cash register was down, they opened their physical ledger, made an entry and gave me a physical handwritten paper receipt written with pen and ink. They said they would make enter the same data online when the online cash register comes back up.
My understanding is that this (NF525) only applies to computerised cash registers (software must be certified NF525 compliant), which there is no obligation to use in the first place.
So it is perfectly legal to use pen and paper and a cash box.
