a. If, in accordance with section three hundred twelve, an agency determines that the use of competitive sealed bidding is not practicable or not advantageous to the city, the agency shall select the most competitive alternative method of procurement provided for by sections three hundred eighteen through three hundred twenty-two which is appropriate under the circumstance. Each agency contract file shall contain documentation of such determination and of the basis upon which each contract is awarded, as is required by the procurement policy board.
b. Each contract for goods, services or construction in value of more than five million dollars proposed by an agency to be awarded which is let by other than (i) competitive sealed bidding, (ii) competitive sealed bids from prequalified vendors, or (iii) competitive sealed proposals, where the weight assigned to each of the factors or criteria to be considered in selecting the proposal most advantageous to the city was set forth in a writing filed in the agency contract file prior to the opening of proposals, shall require the approval of the mayor prior to its execution. Notwithstanding the preceding sentence, the mayor may, where the mayor has determined that it is appropriate, exclude an agency's contracts or a particular category of contracts from the approval requirement of this subdivision.