(iv)any sum paid by the assessee as an employer by way of contribution towards a recognised provident fund or an approved superannuation fund, subject to such limits as may be prescribed for the purpose of recognising the provident fund or approving the superannuation fund, as the case may be; and subject to such conditions as the Board may think fit to specify in cases where the contributions are not in the nature of annual contributions of fixed amounts or annual contributions fixed on some definite basis by reference to the income chargeable under the head "Salaries" or to the contributions or to the number of members of the fund;