A top Senate Republican on Thursday accused the Obama administration of misleading taxpayers about General Motors' loan repayment, saying the struggling auto giant was only able to repay its bailout money by dipping into a separate pot of bailout money.

Sen. Chuck Grassley's charge was backed up by the inspector general for the bailout -- also known as the Trouble Asset Relief Program, or TARP. Watchdog Neil Barofsky told Fox News, as well as the Senate Finance Committee, that General Motors used bailout money to pay back the federal government.

"It appears to be nothing more than an elaborate TARP money shuffle," Grassley, the ranking Republican on the Senate Finance Committee, said in a letter Thursday to Treasury Secretary Timothy Geithner.

GM announced Wednesday that it had paid back the $8.1 billion in loans it received from the U.S. and Canadian governments. Of that, $6.7 billion went to the U.S. treasury.

But Grassley said in his letter that a Securities and Exchange Commission form filed by GM showed that $6.7 billion of the tens of billions the company received was sitting in an escrow account and available to be used for repayment. He called on Geithner to provide more information about why the company was allowed to use bailout money to repay bailout money, and how much of the remaining escrow money GM would be allowed to keep.

"The bottom line seems to be that the TARP loans were 'repaid' with other TARP funds in a Treasury escrow account. The TARP loans were not repaid from money GM is earning selling cars, as GM and the administration have claimed in their speeches, press releases and television commercials," he wrote.

Vice President Biden on Wednesday called the GM repayment a "huge accomplishment."

But Barofsky told Fox News that while it's "somewhat good news," there's a big catch.

"I think the one thing that a lot of people overlook with this is where they got the money to pay back the loan. And it isn't from earnings. ... It's actually from another pool of TARP money that they've already received," he said Wednesday. "I don't think we should exaggerate it too much. Remember that the source of this money is just other TARP money."

Barofsky told the Senate Finance Committee the same thing Tuesday, and said the main way for the federal government to earn money out of GM would be through "a liquidation of its ownership interest."

Grassley criticized this scenario in his letter.

"The taxpayers are still on the hook, and whether TARP funds are ultimately recovered depends entirely on the government's ability to sell GM stock in the future. Treasury has merely exchanged a legal right to repayment for an uncertain hope of sharing in the future growth of GM. A debt-for-equity swap is not a repayment," he wrote.