Whenever wages, rates of pay or any other matters requiring appropriation of moneys by any municipality are included as matters of collective bargaining conducted under the provisions of this article, it is the obligation of the bargaining agent to serve written notice of request for collective bargaining on the corporate authorities at least one hundred twenty (120) days before the last day on which moneys can be appropriated by the municipality to cover the contract period which is the subject of the collective bargaining procedure.
Okla. Stat. tit. 11, § 51-112