When any city of the first class has, prior to March twenty-ninth, one thousand nine hundred and twenty-seven, transferred to the county park commission the care, custody and control of any real estate within the boundaries of the city, and when the county park commission shall determine that it is for the public interest that the care, custody and control of that real estate should be returned to the city, the park commission may by resolution return to such city the care, custody and control of so much of that real estate as may be determined by the park commission to be unnecessary for park purposes, but the area of the real estate which may be returned shall not exceed five acres in extent. The transfer shall not be made unless the governing body of the city by resolution consents thereto.