Lehman Brothers has accused Barclays Capital of taking $US8.2 billion ($A9.51 billion) more than it should have when it bought key assets of the failed investment bank a year ago.
Tuesday was the one-year anniversary of Lehman's chaotic federal bankruptcy filing.
The court approved the sale of its US banking business - estimated to be worth $US1.35 billion ($A1.57 billion) at the time - to Barclays less than a week after it filed.
Now Lehman wants a judge to force Barclays to give back some of the money it took as part of the deal, including $US5 billion ($A5.8 billion) given as extra collateral.
Lehman says the extra value was not disclosed to the court.
Barclays spokesman Michael O'Looney says Lehman is making "an opportunistic claim" now that the economy is beginning to stabilise.