The proceeds from the sale of bonds issued under this subchapter may be applied only for a purpose for which the bonds were issued, except that:
(1) any secured interest received in the sale shall be applied to the payment of the principal of or interest on the bonds sold and, if a portion of the proceeds is not needed for a purpose for which the bonds were issued, that portion shall be applied to the payment of the principal of or interest on the bonds; and
(2) any premium received in the sale of the bonds shall be applied in accordance with § 1201.042(d).