Participation by a political subdivision is entirely voluntary and no political subdivision may be required to sell its bond issues to the public finance authority. Notwithstanding any other state law applicable to the issuance of bonds, a political subdivision desiring to participate in the public finance authority may enter into an agreement with the public finance authority for the purchase by the public finance authority of a bond issue or issues of the political subdivision, including the purchase by the public finance authority of an issue or issues of refunding bonds, which refunding bonds may be required by the agreement to be issued at a rate or rates of interest higher or lower than that of the bond issue or issues to be refunded.