No officer or employee of the city may receive, directly or indirectly from the city of Purcell, or from any person, firm or corporation operating within the city under a public franchise, any service or other valuable thing upon more favorable terms than are granted to the public generally. Any violation of this section shall constitute an offense against this charter and any officer or employee of the city, upon conviction thereof, shall thereby forfeit his office or employment. Nothing herein contained shall prevent such a person, firm or corporation from granting the city such service or other valuable thing free or upon more favorable terms.