The formation of a union as provided in Section 7-2-1 shall not adversely affect the collection of any revenue or the enforcement of any tax or special assessment, levied or assessed in any municipality that has become a borough of a united city. Proceedings to collect revenues and enforce such taxes or special assessments may be instituted and carried on in the name of the municipality. All such revenues, taxes and special assessments 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 or assessed.