While Mr Potato, as he was dubbed by the Mail, turns out to be a middle-management buyer at Sainsbury's, the alleged £3bn bribery and money laundering case over a potato contract with Greenvale cannot be overstated. Rumours of bungs and brown envelopes are par for the course in grocery, as they will be in any industry where large sums of money are at stake. But £3m is a lot of cash to cram in an envelope, even in instalments. And how do you sign off the purchasing order? This isn't a Rolex watch or a family holiday for four in the Caribbean we're talking about. Even Justin King couldn't sign off a £3m expense form claim unchallenged, let alone an operations director at one of his potato suppliers. However, as Duncan Swift argues - not for the first time in these pages - the food and drink industry is particularly prone to extortion and rogue traders. He attributes this to the absence of paperwork in many trading relationships in this industry, together with a lack of effective regulation. Of course, even the current Supermarkets Code of Practice does not permit corruption. But it does permit retrospective discounting, and other 'payments' to retailers, where they may be considered 'reasonable'. And the effect is often the same. OK, the sums gained by an individual may be smaller, but a £3m kickback paid to a supermarket will directly result in a bonus to the buyer. So, while retrospective discounts are expected to be banned in the Competition Commission's forthcoming report into the grocery market, and the search for rogues rightly extended beyond the big four, the fundamentals of the bonus packages that buyers work with leave the system wide open to abuse.