In any county where tolls collected for the use of a bridge or bridges are allocated to the payment of the bonds issued for the construction of said bridge, interest, sinking fund, and the expense of operating and maintaining said bridge or bridges, if the tolls so collected are more than sufficient to meet the payment of said bonds as they severally mature, the interest thereon, the sinking fund, and the expenses of operating and maintaining said bridge or bridges, the board of supervisors may, in their discretion, transfer to the county road and bridge fund any excess of tolls collected over and above the amount necessary to pay the maturing bonds, interest, sinking fund, and expenses of maintaining and operating said bridge or bridges.