Subject to any directions which the Central Government may give in this behalf, any sum of money or part thereof of which the expenditure has been authorised in an estimate for the time being in force sanctioned by the Central Government and which has not been so spent, may at any time be re-appropriated by the Board to meet any excess in any other expenditure authorised in the said estimate:
     Provided that no such reappropriation shall be made from one major head of expenditure to another such head without the previous sanction of the Central Government.