The board may apply all rentals collected or moneys received by it from the lease of unredeemed, unsold parcels, or for the use or occupation of such parcels, to:
(a)
The payment of the incidental expenses of holding and leasing the parcels.
(b)
The payment of any other incidental expenses of, or legal charge against the district.
(c)
The purchase of outstanding district bonds matured or unmatured, with their coupons at not more than the face value of such bonds plus the accrued interest thereon.
(d)
The payment of any call of any operation and maintenance assessment on any unredeemed, unsold parcel theretofore purchased by the county treasurer as trustee for the district.