Whenever a power is granted to or duty is imposed upon a city officer or employee, such power may be exercised or the duty may be performed by a deputy of such officer or employee, or by a person otherwise duly authorized pursuant to law or ordinance, or by an officer of the county, or by a deputy or employee of such officer, when, by contract with the city, such officer is obligated and has agreed to perform certain duties on behalf of the city, unless this code expressly provides otherwise. (2002 Code § 2-4.5).