“Provided further that no deduction shall, in respect of any sum referred to in clause (b), be allowed unless such sum has actually been paid in cash or by issue of a cheque or draft or by any other mode on or before the due date as defined in the Explanation below clause (va) of sub-section (1) of section 36, and where such payment has been made otherwise than in cash, the sum has been realised within fifteen days from the due date.”