And the banks financing other buyouts have grown more assertive in demanding better terms. The bankers that committed to finance the $26 billion buyout of First Data won some small concessions to the debt structure after weeks of pleading.

But the deal for Sallie Mae may be the most delicate of all. That has been reflected in the company’s stock price, which has fallen after reaching a 52-week high of $58. Shares of SLM closed yesterday at $48.55, because investors expect a price cut.

Steven M. Davidoff, an assistant professor of law at Wayne State Law School, said the buyers would have two options if they wanted to get out of the deal, but neither was guaranteed to succeed. They could claim that the new legislation represents a material adverse change, though, Mr. Davidoff said, it may be a hard case to make. The language in the contract between the buying group and Sallie Mae explicitly contemplated the new bill, though at the time the deal was struck, the legislation was expected to be less onerous.

In a presentation to its investors in May, J. C. Flowers said the legislation “could trigger a material adverse effect” if it was worse than originally expected.

In a statement earlier this month, Sallie Mae, responding to speculation the deal could collapse, said, “Sallie Mae estimates the adverse impact of existing legislative proposals on projected 2008-2012 net income to be less than 10 percent as compared to the matters already disclosed to the buyer. Under applicable legal standards, this impact would not constitute” a material adverse change.

According to Mr. Davidoff, playing what is known as the MAC, or material adverse change, card, “would be a crapshoot. The Delaware courts generally haven’t looked at this issue.”

But perhaps more likely, the buyers may decide simply to threaten to walk away.

That, however, comes with its own complications. One big risk is not price but reputation. Even more than JPMorgan and Bank of America, J. C. Flowers has much to lose should the buyout falter. Since his start at Goldman Sachs, Mr. Flowers has built a reputation as a shrewd deal maker. But the financial services community is a small one, and backing out of the deal could make it difficult for him to pursue other deals.