A state agency or a local public body shall be entitled to audit the books and records of a contractor or any subcontractor under any negotiated contract or subcontract other than a firm fixed-price contract to the extent that such books and records relate to the performance of such contract or subcontract. Such books and records shall be maintained by the contractor for a period of three years from the date of final payment under the prime contract and by the subcontractor for a period of three years from the date of final payment under the subcontract unless a shorter period is otherwise authorized in writing.