(b) Where all or part of the indebtedness secured by a mortgage of real property within any city in the state having a population of one million or more has been paid and new funds are advanced or re-advanced which are to be secured by such mortgage, the contract or agreement by which such funds are advanced or re-advanced shall be deemed a mortgage of real property for purposes of this article, and shall be taxable as such upon the amount of such new funds, except as otherwise provided in section two hundred fifty-three-b of this article. 3. The term "tax district" as used in this article means a city or town.