Where any municipal sewer improvement district has fully retired and paid out the bonded indebtedness of the district and there remains a surplus of money derived from the collection of benefit assessments levied against the real property in the district for the purpose of retiring the bonds or funds derived from the sale of unredeemed real property acquired by the district, the surplus funds may be transferred to the municipality where the municipality has taken over the sewer improvement constructed by the district and is maintaining it.