40 ILCS 5/6-206
If at any time the balance in the expense reserve, the prior service annuity reserve, the child's annuity reserve, the duty disability reserve, or the ordinary disability reserve (either one of these) is not sufficient to provide for expenses, annuities or benefits which are chargeable to such reserves, the remainder required shall be transferred from any or all of the following named reserves in the order stated: city contribution reserve, prior service annuity reserve, salary deduction reserve. When amounts in excess of that required to pay any expenses, annuities or benefits chargeable to the reserves to which such sums have been transferred shall be received into such reserves, such excess amounts shall be transferred to the reserves from which any such sums were taken until the full sum is returned to the reserves from which a transfer was made. Interest at 4% per annum upon any transfer and retransfer shall be credited to the investment and interest reserve.
40 ILCS 5/6-206