Initial contributions—Subject to any condition which the Board may think fit to specify under clause (iv) of sub-section (1) of section 36, the amount to be allowed as a deduction on account of an initial contribution which an employer may make in respect of the past services of an employee admitted to the benefits of a fund shall not exceed twenty-five per cent of the employee's salary for each year [up to the 21st September, 1997 and after 21st September, 1997, twenty-seven per cent of the employee's salary for each year] of his past service with the employer as reduced by the employer's contribution, if any, to any provident fund (whether recognised or not) in respect of that employee for each such year