REFUNDS FOR AVIATION-GRADE KEROSENE NOT USED IN AVIATION- If tax has been imposed under section 4081 at the rate specified in section 4081(a)(2)(A)(iv) and the fuel is used other than in an aircraft, the Secretary shall pay (without interest) to the ultimate purchaser of such fuel an amount equal to the amount of tax imposed on such fuel reduced by the amount of tax that would be imposed under sec