Americans give the government interest-free loans with their salary withholdings. There's a way to turn the tables.

In what could be called a free loan from the Social Security Administration (SSA), savvy investors have found a way to maximize the federal benefit by telling the government to stop sending the monthly checks, repaying what has already been received and, years later, refiling their claim.

The strategy can potentially yield big profits if the money is invested, according to the Center for Retirement Research at Boston College. Though, of course, all of it could be lost.

There's a little-known provision that allows individuals already collecting Social Security to "change their minds and start over" by filing Form SSA-521, the report said. For example, a retiree can claim Social Security at age 62, halt payments and reclaim at age 70, and receive a higher benefit, provided he has paid back the money previously received. That could produce as much as $14,000 more a year as payments are spread out in accordance with life expectancies.

In essence, the claimant is a borrower required to pay back only the principal. The payments could, over time, be invested with the interest pocketed. Retirees could sock the money in a Roth IRA and invest in stocks. That way there would be no withdrawal or tax penalty. Traditional IRAs and money market accounts would be other safe resting places for the money to grow. If a retiree followed this strategy earlier this year by putting money near the bottom of the stock-market rout, he could have doubled or tripled returns in a few months. Citigroup's ( C) stock quadrupled in two months, while Bank of America ( BAC) and General Electric ( GE) also surged. Simply investing in the S&P 500 Index in early March would have generated a gain of about 30%.

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