The U.S. Postal Service has overfunded its federal pension obligations by nearly 105 percent, or $13.1 billion, for the federal fiscal year ending Sept. 30, 2011, according to a report issued last week by the agency’s inspector general.

The inspector general suggested reverting the surplus money to the Postal Service. But the Office of Personnel Management is not permitted to return surplus contributions to federal agencies. That would require an act of Congress.

“We believe allowing the Postal Service to use its current and future surplus funds will provide significant financial relief and dramatically improve cash flow,” the inspector general noted.

The Postal Service, which lost $3.2 billion in the quarter that ended March 31, could use the money. The agency has lost $17 billion in the past three years, according to the inspector general report.

To alleviate the financial impact, Postmaster General Patrick Donahoe has proposed several controversial measures that have stoked the ire of letter carriers and legislators alike. He has proposed eliminating Saturday delivery, consolidating or closing mail-processing plants and reducing the hours of thousands of employees at rural post offices. In May, the Postal Service offered early retirement buyouts to nearly 45,000 mail handlers.

And the billions in overfunding would help the Postal Service’s bottom line considerably.

“The pension overfunding we absolutely think should be returned to the Postal Service to help them get out of the hole that they’re in and get some stability,” said Sally Davidow, communications director for the American Postal Workers Union.

A provision in the 2012 postal reform act that passed the Senate and is currently in the House would return the projected $11.4 billion surplus that the agency contributed to the Federal Employee Retirement System. That bill also proposes returning all future surpluses to the agency.

A separate postal reform bill introduced in the House would have postal workers make the same retirement contributions as other federal employees and would return a $10 billion 2010 FERS surplus within two weeks after passage. Both bills are pending House approval.

Further hurting the Postal Service’s finances, according to the inspector general’s report, is a 2006 postal reform act that requires the agency to prefund the health plans of future retirees through 2016. Prefunding is sucking more than $5 billion a year off the Postal Service’s ledger books.

The inspector general proposed allowing the Postal Service to draw off its extensive property holdings, assessed at $85 billion, as collateral to address any retirement and health obligations if unexpected problems were to arise.

“The $85 billion would sufficiently cover the remaining unfunded retiree health-care benefit obligation of $46 billion,” the report said.

Joseph Corbett, the Postal Service’s chief financial officer, in comments included in the report, said he hopes to have the surplus pension issues resolved by the end of the year.

--A previous version of this post incorrectly reported the amount of overfunding and the date the inspector general’s report was issued.