King of pay: Wal-Mart assistant treasurer Mike Cook decides what technology gets used in Walmart checkout lines.

Many technology companies think mobile phones will revolutionize how we pay at retail stores. For that to happen, they’ll have to make it past the desk of Mike Cook in Bentonville, Arkansas.

And Mike’s a little skeptical.

Cook, 47, is the person who decides how people pay at Wal-Mart Stores, the world’s largest retailer. A small-town Missouri native with a forgettable title (VP and assistant treasurer), Cook directly oversees the collection of nearly half a trillion dollars in cash, credit and coupons from customers around the globe each year.

That’s more than the GDP of Sweden and explains why so many technology companies are now courting Wal-Mart: a deal with the giant retailer could make the fortunes of any company hawking a new form of payment.

But Cook says he isn’t necessarily looking for a digital revolution. He is looking for ways to save Wal-Mart and its customers some money: “A lot of [what I do] is thinking about how to make big and small improvements on our volume that come out to significant dollar savings.” A 20-year veteran who has worked at Wal-Mart’s headquarters for so long he has trouble remembering his first job (assistant cash manager), he calls payments “the most complex simple game on earth.”

“I think it’s probably boring to most people, but I find it invigorating,” says Cook.

And it’s getting more complicated. Wal-Mart is now in the middle of a brewing fight over who will control payments made from smart phones. Such mobile payments are expected to soar to $670 billion globally by 2015—and companies such as PayPal, Google, and Verizon are jockeying to provide mobile-phone wallets that will give them leverage over fees, as well as detailed data about consumers’ shopping habits.

Breaking into Wal-Mart’s network could help any one of them dominate. This month, however, the Wall Street Journal reported that Wal-Mart, Target, and a dozen other big retailers are joining together to study the possibility of a merchant-led mobile wallet that would compete with those from tech companies.

Cook, who confirmed the broad outlines of the plan, says Wal-Mart has doubts about the technology firms’ intentions. “Would you want some telco provider or some technology provider knowing all of your prescription goods that you purchased at Walgreens or Walmart?” he says. “It’s not as though we’re not excited about mobile. It’s that we’re not excited about the solutions that are out there.”