Student Loan and Van Ru

Shortly after my husband died, I was contacted about a 20-year-old student loan in fed default. It is my loan, but I thought it had been paid off 15 years ago. That's what my husband told me (before he died, of course). I made a telephone settlement arrangement with Van-Ru. I agreed to 10 monthly auto-withdrawals from my account of $250, after which time the loan would be out of default and the payments would go down to $50 per mo. Once out of default, I would have the opportunity to research and file a dispute. After taking 5 payments, Van Ru sent me a contract and said I MUST sign which listed entirely different terms. The contract says I agree and understand that the payments will go UP after 9 payments. AND that I agree to pay additional interest. I called the rep and she was outrageously rude. She refused to discuss the contract. Said sign it or else. I asked why the terms changed and she said, "We don't have time to discuss every little term on the contract with you before we accept your payments. Stop whining, we are offering you an opportunity here." What can I do?