Depending on country and exact POS setup, they might not be able to take cash if POS is down. For example, in Poland, your typical restaurant or shop needs to

Depending on country and exact POS setup, they might not be able to take cash if POS is down. For example, in Poland, your typical restaurant or shop needs to generate tax receipt (as well as properly calculate the tax), and uses either a separate receipt POS device, or POS with appropriate receipt printer (the devices are certified and for example do simultaneous two prints - one for client one for seller - or use digitally-signed storage for seller copy). If the POS isn't designed properly to operate in case of network failure... welp, can't take cash either, at least not legally.