Whenever the board of supervisors determines that any real property, or interest therein, acquired by the county for highway purposes is no longer necessary for those purposes, the county may sell or exchange the real property, or interest therein, in the manner and upon the terms and conditions approved by the board of supervisors. Any conveyance under this section shall be executed on behalf of the county by the chairman of the board of supervisors, and the money received for the real property shall be paid into the county treasury to the credit of any fund available for highway purposes, which the board of supervisors designates.