The laws are there due to anti-money laundering. Since virtually no cash income (in those country) is viable - think salaries, etc., it's not hard to be followed. All retail sales need a valid VAT receipt, all of those receipts have source of the payment as well. Wholesale tends to be never in cash (requires tax administration approval), the laws might differ, of course - however that's a common framework.

So it's sort of automatic.

>Any source for this?

Generally, should be trivial search info: e.g. "cash payment amount greece" produces multiple decent results