(a) Except as provided in subdivision (b), the assets of the plan shall never inure to the benefit of an employer and shall be held for the exclusive purposes of providing benefits to members and beneficiaries of the Defined Benefit Program as well as the participants and beneficiaries of the Cash Balance Benefit Program and defraying reasonable expenses of administering the plan and the system.
(b) In the case of a contribution that is made by an employer by a mistake of fact, subdivision (a) shall not prohibit the return of that contribution within one year after the system knows, or should know in the ordinary course of business, that the contribution was made by a mistake of fact.