Where any part of a city adjoins or is included in a metropolitan district organized under the provisions of Act No. 312 of the Public Acts of 1929, as amended, being section 119.1 to 119.15, inclusive, of the Compiled Laws of 1948, such city may acquire by purchase the water supply system and/or sewage disposal system owned by such metropolitan district, and may own, maintain and operate the same either separately or as a part of its own water supply system and/or sewage disposal system, and may improve, enlarge and extend the same. Any such metropolitan district is hereby empowered to sell its water supply system and/or sewage disposal system in accordance with the terms of this act. In the event that there are outstanding any bonds pledging the revenues of such metropolitan district system or systems, the city may assume the obligations of the metropolitan district on said bonds as a part or complete payment of the purchase price. The purchase agreement shall be evidenced by a contract between the city and metropolitan district, which contract shall be approved by a resolution adopted by the affirmative vote of 3/5 of the members elect of the respective governing bodies of such city and metropolitan district. Such a contract shall contain those provisions which the parties thereto shall deem necessary to properly effect such sale and purchase and to define the rights and obligations of the parties. No such contract shall be entered into unless the disposal of the system or systems shall have been authorized by 3/5 of the electors of the metropolitan district voting thereon at a regular or special election and unless the acquirement of the system or systems shall have been authorized by 3/5 of the electors of the city voting thereon at a regular or special city election. Such an authorization shall be valid for the purpose of this act if made at an election held after the effective date of this act or within 1 year prior thereto. The foregoing requirements as to authorization by the electors shall supersede any charter requirements in that respect.