India's state coffers are $10 billion richer thanks to the unexpected success of a government-led income declaration scheme. But among the many areas in need of urgent investment, from farmers' welfare to job creation schemes, Prime Minister Narendra Modi faces a difficult decision on how to spend the cash.

From June to September, Modi's administration sent about 700,000 notices to suspected tax dodgers, persuading them to reveal undeclared income and assets, referred to as black money, and pay a penalty. In return, the tax evaders would not be prosecuted, the government promised.