Where a tax rate or rates have been authorized by referendum in any municipality that has become a borough of a united city as provided in Section 7-2-1, proceedings to levy, collect and enforce such taxes in all boroughs of the united city may be instituted and carried on in the name of the united city. All such taxes that are collected shall be paid over to the treasurer of the united city, but they shall be used for the purpose for which they were levied in all of the boroughs of the united city.