No expenditure or contract for the expenditure of state public funds for purposes of construction of highway bypasses or highway relocation projects diverting public motor vehicle travel from a previously existing highway route shall be made that violates the provisions of an existing agreement between a municipality or county and the state transportation commission relating to obtaining the consent of the municipality or county pursuant to the provisions of this section. An existing agreement between a municipality or county and the state transportation commission may be amended or revised by mutual consent of the parties to the agreement. Once authority is given by the governing authority and the state transportation commission has affirmatively acted in reliance upon the expressed approval, public funds may be expended and contracts executed despite subsequent withdrawal of approval by the governing authority.