That Citigroup employee Andrew Hall is an exceptional energy trader is not in doubt. But should he really be paid a $100 million bonus for last year's work from the bank that was saved by the American taxpayer?

Few outside the rarified dealing rooms of the City and Wall Street think he should. But clawing his pay down is proving tough for Barack Obama's pay czar.

Under the terms of the US banks bailout, the government was supposed to be able to curb giant pay deals which are, essentially, coming out of the taxpayer's pockets. But this does not count in the case of Hall, who earned massive profits for the bank on the volatile energy markets of 2008.

The problem is that Hall's contract was signed before a cut-off date of 11 February and therefore is not caught in the net of pay watchdog Kenneth Feinberg.

Now Hall's pay is rapidly turning into a major potential political headache for the President, with growing pressure from Democrats to extend Feinberg's reach.

“Congress and taxpayers are rightly alarmed that companies would enrich their executives while deferring repayment of the federal financial assistance that helped them avoid financial catastrophe,” said House of Representatives speaker Nancy Pelosi and Barney Frank, both Democrats, in a letter to US treasury secretary Tim Geithner.

Some now expect Obama to extend Feinberg's far-reaching powers to include Hall, but such a deal would force Citi to break its own contracts with the employee.