For law firms advising the Treasury Department on the Troubled Assets Relief Program, it’s one thing to win a fat government contract for legal services — but another to actually see the cash.

The TARP watchdog, the Congressional Oversight Panel, released a report (pdf) Thursday detailing exactly how much Treasury has shelled out to the 18 law firms that have been awarded contracts since the program began in late 2008. In most cases, the difference between the potential contract value and the amount owed for services performed is significant.