For purposes of the deduction allowable under section 29(1)(a), subject to any condition which the Board may think fit to specify, the initial contribution of the employer to a superannuation fund for the past services of an employee shall not exceed the total of 27% of the salary of the employee for each year of past service, as reduced by the contributions of the employer to a provident fund (recognized or unrecognized), if any, for the same employee for each such year