Liabilities accruing under the retirement system because of benefits other than such as are the equivalent of contributions by the members, with accumulated interests, shall be met by contributions by the district. Prior service or other liabilities of the district may be met by annual appropriations instead of by one appropriation for the total of the liabilities; but until the present value of regular contributions for current service, together with assets then available, equals the present value of all allowances and benefits granted or to be granted under the system, the appropriation for any one year when added to any unused balance of any previous appropriations for

such purpose shall not be less than the amount disbursed during that year on account of prior service or other liabilities of the district.