WASHINGTON – The planned $25 billion buyout of SLM could be in jeopardy as the investors that agreed to buy the nation's largest student lender, commonly known as Sallie Mae, say congressional legislation could kill the deal. Sallie Mae disputes that. The takeover deal, one of the largest private buyouts ever, is led by private-equity firm J.C. Flowers. At issue are the two sides' interpretation of their acquisition agreement, signed in April, under which significant negative developments can nullify the deal. Legislation that passed the House Wednesday would cut in half the interest rate on government-backed student loans – a move that would be paid for by a roughly $19 billion reduction in federal subsidies to student lenders like Sallie Mae. The bill also would cap annual loan repayments at a percentage of a student's income, protecting graduates with low salaries from having to pay back more than they can afford.