'(2) The Minister shall not so exercise the powers conferred by subsection (1) of this section or by section 4(1) of the Transport Act 1974, that the amount, or the aggregate amount, of principal which he or she may at any one time be liable to pay pursuant to a guarantee or guarantees under this section or section 4 of the Transport Act 1974 (or under both those sections), and for the time being in force, together with the amount of principal (if any) which he or she has previously paid pursuant to any such guarantees and which has not been repaid, exceeds €600,000,000.'.".