It shall be lawful for cities of the first class in this Commonwealth to sell the bonds of such cities to the sinking-fund commissioners thereof, without advertisement, at not less than par, whenever, in the opinion of the mayor, the city controller, and the city solicitor, or a majority of them, such sale without advertisement may be deemed necessary or advisable; and the said sinking-fund commissioners shall thereupon be duly authorized to purchase the said bonds as aforesaid.