Section 10A. In special elections, called for the purposes of filling a vacancy in the general court, state funds for compensation of members pursuant to section nine of chapter three of the General Laws not expended during such period of vacancy shall not revert to the General Fund or any legislative accounts but shall be set aside in a special fund in the office of the state treasurer and made payable to the city or town in which such election was held. The city or town clerk shall certify to the state treasurer the total actual costs of holding such special election, and such city or town shall be reimbursed up to one hundred percent of the net costs thereof from said fund without further appropriation; provided, however, that the state treasurer shall not authorize any expenditure which would cause said fund to be in deficit. ''Net costs'' for purposes of this section shall mean the total actual costs as certified by the city or town clerks less any other reimbursements available to such city or town.