Even if it was their "fault"--i.e. they forgot to charge or bill the full amount--if the charges are lawful--that is, the borrower legitimately owes them--then the lender can require they be paid all at once; that is, they can require that the entire shortfall be made up immediately. It is voluntary on their part to accept payment over time; therefore, you cannot force them to give you more time than a year, though you an certainly ask for it and see if they will agree.