Apple avoided as much as $9.2 billion in taxes by financing part of a $55 billion stock buyback with debt rather than offshore cash that would have been billed by the U.S. government, Moody's Investment Services estimates.

Based on current rates, Apple will pay interest of about $308 million a year on the $17 billion bond offering, said Gerald Granovsky, a senior vice president at Moody's.

"From a pure corporate-finance theory perspective, this was a no-brainer," Granovsky said.

If the funds had come from Apple's offshore cash pile of about $100 billion, the Cupertino-based iPhone maker would have had to pay a 35 percent tax to repatriate the money, Granovsky said. That means Apple avoided about $9.2 billion in taxes. And since interest payments are tax-deductible, that's another $100 million a year, Granovsky said.

In fiscal year 2012, Apple paid $6 billion in federal corporate income taxes, which is 1 out of every 40 dollars in corporate income taxes collected by the U.S. government, said Steve Dowling, a company spokesman.

"That makes Apple one of the top corporate income tax payers in the country, if not the largest," he said.

Chief Financial Officer Peter Oppenheimer said on an April 23 conference call that incorporating debt into the capital structure will provide benefits, including access to attractively priced capital, a reduction cost of capital and an efficient leverage of the balance sheet.

Apple's sale of bonds this week was the biggest corporate offering on record.

The stock has dropped 36 percent since its September record, partly on concerns that the company may not quickly unveil blockbuster hits like the iPhone or iPad up.

The company said last week that it would return an additional $55 billion in cash to shareholders through share repurchases and increased dividends, to compensate for a stock that's been hammered by signs of slowing growth. In total Apple will have returned $100 billion, including past buybacks and dividends, through 2015.

The Financial Times earlier reported that Apple will avoid a potential tax bill of as much as $9 billion using the proceeds from the bond sale.