The board of supervisors of any county may, by a unanimous vote, expend any portion of the moneys in any of the funds named in section 5156 for the payment of principal or interest, the redemption or purchase of bonds of the county or of a special assessment district within the county if special assessments have been or are to be levied for such payments, and the proceeds of the bonds have been used for the acquisition of lands or the improvement or maintenance of lands or public parks within the county.