Any dividends hereafter declared or rate reductions hereafter made or continued for the first or any subsequent contract year under any group policy or group annuity contract heretofore or hereafter issued, may be applied to reduce the employer's part of the cost: Provided, That if, at any time, under a policy or contract, towards the cost of which the employes contribute, the aggregate of any dividends or rate reductions so applied is in excess of the employer's share of the aggregate cost, such excess shall be applied by the employer for the sole benefit of the employes.