The board of supervisors may, by a four-fifths vote, enter into an agreement with the lessee or concessionaire to amend any existing lease, sublease, concession, or managerial contract relating to improved property entered into pursuant to Section 25536 to permit the permanent improvement or alteration of the county-owned, -leased, or -managed property at the expense of the lessee or concessionaire and to permit a credit on rentals or other reimbursement to the extent thereof during the remainder of the lease, sublease, concession, or contract.