The proceeds of the bonds of each issue may be used only for the payment of the cost of the industrial project or medical project for which the bonds were issued, and shall be disbursed in the manner and under any restrictions provided in the resolution authorizing the issuance of the bonds or in the trust agreement securing the bonds. Any proceeds of the bonds of an issue that exceed the cost of the project for which the bonds were issued shall be deposited to the credit of the sinking fund for those bonds.