§ 64. Commutation for bridges. The commissioner of transportation may commute with owners and claimants of bridges over any canal, by paying such owner or claimant such sum in lieu of a bridge as may be agreed on between the claimant and the commissioner of transportation. If, in the opinion of the commissioner of transportation, a bridge should not be rebuilt, and the amount to be paid be not agreed upon, the bridge shall not be built, but the damages sustained by such owner by being deprived of such bridge and which the state under all the circumstances ought of right to pay, shall be ascertained in the same manner as damages for the appropriation of real property, for the use of the canal and paid by the commissioner of transportation, on the approval of the attorney-general. If the damages claimed are for the deprivation of a bridge which the claimant had before constructed or maintained, the circumstance of his being equitably bound to contribute proportionately toward the construction and maintenance of an enlarged bridge shall be taken into consideration and a proper amount of that account shall be set off against any damage to which the claimant might otherwise be entitled.