Notwithstanding any contract to the contrary, where any firm is liable to pay any tax (including any penalty) under this Act, the firm and each of the partners of the firm shall be jointly and severally liable for such payment:
     Provided that where any such partner retires from the firm, he shall intimate the date of his retirement to the Commissioner by a notice in that behalf in writing and he shall be liable to pay tax (including any penalty) remaining unpaid at the time of his retirement and any tax (including any penalty) due up to the date of his retirement though unassessed on that date:
     Provided further that if no such intimation is given within fifteen days from the date of retirement, the liability of the partner under the first proviso shall continue until the date on which such intimation is received by the Commissioner.