Chapter 171, Local Government Code, applies to a board member of an authority, except that an authority created before 1980 in which the principal municipality has a population of less than 1.9 million may not enter into a contract or agreement with a business entity in which a board member or the general manager owns five percent or more of the voting stock or shares of the entity or receives funds from the entity exceeding five percent of the member’s or general manager’s gross income. A contract executed by an authority in violation of this section is voidable.