No agreement negotiated by the director under Section 1870 that reduces any liability by ten thousand dollars ($10,000) or more shall be effective until it is reviewed and approved by the Unemployment Insurance Appeals Board. In reviewing any agreement, the Unemployment Insurance Appeals Board shall be restricted to determining, based on the case file submitted by the director, whether the director has exercised due diligence in determining whether the conditions set forth in subdivision (a) of Section 1870 have been satisfied.