In order that any boundary line bridge or boundary line toll bridge may be added to the highway systems of the State of Nebraska and an adjoining state, the department shall have the power and authority to enter into agreements with an adjoining state to determine the manner and method of purchasing any such bridge, and the land, real or personal property, or any easement or interest necessary thereto, through the joint action of the department and the adjoining state, from the person holding title thereto. Such agreements may provide for any reasonable means of purchase and acceptance of title by the department and the adjoining state not in conflict with the laws of the State of Nebraska and the adjoining state; Provided, that no such agreement shall authorize, nor be construed to authorize, (1) the department to purchase the whole of any boundary line bridge or boundary line toll bridge, and the land, real or personal property, or easement or interest necessary thereto, nor (2) to complete any contract for the purchase of that portion of any such bridge, and the land, real or personal property, or easement or interest necessary thereto, located within the State of Nebraska which is not conditioned in performance upon the adjoining state completing a binding contract for purchase of that portion of such bridge, and the land, real or personal property, or easement or interest necessary thereto, located within the adjoining state with the person holding title thereto.