PGI 242.7100 authorizes voluntary refunds iaw the following:

(3) Request voluntary refunds only when—

(i) The contracting officer concludes that the contractor overcharged under a contract, or inadequately compensated the Government for the use of Government-owned property, or inadequately compensated the Government in the disposition of contractor inventory;

You did not indicate after follow-up that your situation differed from the conditions above.