(a) A resolution or an order authorizing the issuance of obligations under this subchapter shall provide for the creation of a sinking fund which shall include sums fully sufficient to pay principal of and interest on the obligations. Money deposited in the sinking fund shall be taken from revenue pledged for the payment of the obligations and shall be deposited in the fund as the revenue is collected.
(b) The money in the sinking fund shall be applied solely to the payment of interest on the obligations for the payment of which the fund is created and for the retirement of the obligations at or before maturity in the manner provided by this subchapter.

(c) The commission, at the time obligations are authorized under this subchapter, may provide that all money in the sinking fund which is in excess of the amount required for the payment of the principal of and interest on the outstanding obligations, for a period of time it may determine, shall be spent once each year pursuant to the commission’s orders for the purchase of obligations, if any can be purchased at a price the commission finds reasonable, for the account of which the sinking fund has been accumulated.
(d) If the obligations contain an option permitting retirement before maturity, the commission may provide that the excess sums shall be paid out as authorized by Subsection (b) of this section for the purchase of the obligations, but if the commission is unable to purchase sufficient obligations of the issue to absorb all the surplus, it shall call a sufficient amount of the obligations for redemption to absorb insofar as practicable the entire surplus remaining in the sinking fund.
(e) The commission may provide that any excess in the sinking fund which cannot be applied to the purchase or redemption of obligations shall remain in the sinking fund for payment of principal and interest and for subsequent call for purchase or redemption.