The principal proceeds from sales of City-owned, non-trust lands shall be recorded in a separate City account. Monies in this account shall be invested and may not be spent or appropriated for any purpose. Revenues earned on monies in this account may be used for any lawful purpose. For this purpose, non-trust lands are defined as lands owned by the City other than lands held in trust by the City under the terms of conveyance thereof to the City. Proceeds from sales of lands received on foreclosure for special assessments or for taxes shall first be reduced by necessary amounts for payments of such assessments, taxes, and related costs.