One reason for tech companies’ optimism is that digital payments in India have increased over the past year. The value of transactions using digital wallets, the business on which Paytm was built, rose 64 percent from December 2016 to December 2017. Transactions made with the Unified Payments Interface, a government-backed technology used by Tez and many other mobile apps, went from virtually nothing a year ago to $2.1 billion last month.

Leading India’s budding payments shift are Paytm and its chief executive, Vijay Shekhar Sharma.

Mr. Sharma founded the company seven years ago as a way for cellphone users to pay their bills online. It is now India’s largest consumer-payments app, with 302 million account holders and 90 million active users.

Customers can use it to buy goods at physical stores, book movie or airline tickets, send money to each other or order items from Paytm’s online mall. A transaction requires a quick scan of a merchant’s bar code or a few taps on a smartphone, rivaling Apple Pay or Venmo in simplicity.

Mr. Sharma aspires to put his company at the center of Indians’ financial lives, and he has pledged to spend $1.9 billion over the next two years toward that goal.

“Our truest ambition is for Paytm to be known as the bank for this new-age, digital, mobile world,” he said in an interview at the company’s headquarters in Noida, just outside Delhi.

Merchants like Mr. Singh are crucial to Paytm’s plans.

The company pays Mr. Singh a bounty of 20 rupees for each of the eight or so customers he signs up each month, with additional payments if a newcomer continues to use the service. He earns an additional 18 rupees each time he verifies the identity of an existing Paytm user with his fingerprint scanner, a new requirement imposed by the government on all digital wallet companies.