Should the Secretary of Highways so elect, he may enter into an agreement or agreements with any State authority or agency so authorized to act, whereby such authority or agency can acquire, for the use of the Commonwealth, any toll bridge located wholly within Pennsylvania. The terms of any agreement entered into pursuant to this section shall include provisions whereby any bridge so acquired shall become a free bridge whenever the tolls collected therefrom, less the expense of toll collection, shall be sufficient to retire all bonds issued by the authority or agency for such acquisition, or whenever sufficient funds shall be provided for such purpose, either by the sale of bonds pursuant to the provisions of the constitutional amendment authorizing the issuance of bonds for the acquisition of toll bridges within Pennsylvania, or by statutory appropriation as duly authorized by law.