Korea made cash traceable by paying consumers to register every purchase

In the early 2000s Korea's shadow economy was estimated near a quarter of GDP, and even after a 1999 income-tax deduction pulled spending onto credit cards, cash still covered about half of private consumption by 2004 — sales merchants could keep off the books. On January 1, 2005, the National Tax Service extended the same buyer-side logic to cash itself with the Cash Receipt System.

The design is elegant because it recruits the honest side of every transaction. A consumer registers once — online in minutes — and then gives a membership card or phone number whenever paying cash. The sale is transmitted directly to the tax authority, and at year-end the taxpayer deducts 20 percent of registered cash spending above a threshold of 20 percent of income, up to a cap. The reporting threshold was lowered from 5,000 won to 1 won in 2008, so even the smallest purchase became worth registering. Every receipt a customer claims is a sale the merchant cannot deny.

The incentive reversed the usual cat-and-mouse: instead of inspectors chasing merchants, customers chase receipts. Nearly two decades later the system remains embedded in Korean tax life — having survived repeated sunset debates — and 'Do you want your cash receipt issued?' is the standard question at every till.

Give the buyer a tax break to report the seller's cash sale

It enlists millions of consumers as reporters, each with a personal financial motive, instead of a handful of auditors.

The deduction makes demanding a receipt the default behavior, so evasion requires the buyer's active cooperation.

It reuses existing card-terminal infrastructure, so merchants join without new equipment.

Dropping the registration threshold to 1 won removed any purchase too small to bother reporting.

You don't have to audit sellers if buyers profit from reporting: a small deduction on the payer's side creates a self-enforcing paper trail across the cash economy at a fraction of enforcement cost.

Two decades on, the cash receipt system and the credit-card deduction remain core tools of Korea's tax administration and are studied as a model for formalizing cash economies; the IMF and tax researchers cite them as a successful use of consumer-side incentives to shrink the underground economy.