This section and Section 39-5-18 N.M. Stat. Ann. do not apply to any foreclosure sale made before the effective date of this section. The parties to any such instrument may, by its terms, shorten the redemption period to not less than one month, but the district court may in such cases, upon a sufficient showing before judgment that redemption will be effected, increase the period of redemption to not to exceed nine months notwithstanding the terms of such instrument.