If the owner or owners of such bridge accept such offer, or whenever the Secretary of Highways and the owner or owners of such bridge shall agree on a purchase price after negotiation, the Secretary of Highways and said owner or owners shall enter into a written agreement by the terms of which the owner or owners shall transfer to the Commonwealth, at and for the price agreed upon, a good, sufficient and marketable title to such toll bridge, as herein defined, or transfer or procure the transfer to the Commonwealth of all of the outstanding stock of the owner corporation, as provided in section five of this act.¹