The agency counsel relieves a certifying official from liability for transportation overpayments in cases where—

(a) Postpayment is the approved method of auditing;

(b) The overpayment occurred solely because the administrative review before payment did not verify transportation rates; and

(c) The overpayment was the result of using improper transportation rates or freight classifications or the failure to deduct the correct amount under a land grant law or agreement.