The tax payable under this Act by any person earning salary or wages shall be deducted by his employer from the salary or wages payable to such person before such salary or wages is paid to him, and such employer shall, irrespective of whether such deduction has been made or not, when the salary or wages is paid to such persons, be liable to pay tax on behalf of all such persons :

Provided that if the employer is an officer of the State Government or the Central Government, the manner in which the employer shall discharge the said liability shall be such as may be prescribed :

Provided further that where any person earning a salary or wages is simultaneously engaged in employment of more than one employer, and such person furnishes to his employer or employers a certificate in the prescribed form declaring, that he shall pay the tax himself, then the employer or employers of such person shall not deduct the tax from the salary or wages payable to such person and such employer or employers, as the case may be, shall not be liable to pay tax on behalf of such person.