The trustees of any group self-insurers' fund may not allow advance premium discounts to any member in excess of that allowed by the excess insurance underwriter, subject, however, to a maximum of 15 percent of their standard premium. Any surplus monies for a fund year in excess of the amount necessary to fulfill all obligations under the Workers' Compensation Law for that fund year, including a provision for claims incurred but not reported, may be declared to be refundable by the trustees at any time, and the amount of the declaration shall be a fixed liability of the fund at the time of the declaration. The date of payment shall be as agreed by the trustees except that surplus monies not needed to satisfy the loss fund requirements, that is, trustees' funds, may be refunded immediately after the end of the fund year with the approval of the Commission. The intent of this section is to ensure that sufficient monies are retained to assure that total assets are greater than total liabilities for each fund year. In the event of a deficit in any fund year, the deficit shall be made up immediately from any of the following: Unencumbered surplus from a fund year other than the current fund year; Trustees' funds; By assessment of the membership of the deficit fund year if ordered; or By such alternative method as the Commission may approve. The Commission shall be notified before any transfer of unencumbered surplus funds.