The Central Government may, after due appropriation made by Parliament by law in this behalf, advance to the Exim Bank--
         (a) a loan of twenty crores of rupees at a rate of interest of five and a quarter per cent. per annum repayable in fifteen equal annual instalments, commencing on the expiry of a period of fifteen years from the date of receipt of the loan; and
         (b) such further sums of money by way of loan on such terms and conditions as may be agreed upon:
         Provided that the Central Government may, on a request being made to it by the Exim Bank, increase the number of instalments or alter the amount of any instalment or vary the date on which any instalment is payable under clause (a).