(a) If a subject or medium of payment of a contract, security, or instrument is a currency that has been substituted or replaced by the euro, the euro shall be a commercially reasonable substitute and substantial equivalent that may be either (i) used in determining the value of that currency or (ii) tendered, in each case at the conversion rate specified in, and otherwise calculated in accordance with, the regulations adopted by the Council of the European Union.
     (b) If a subject or medium of payment of a contract, security, or instrument is the ECU, the euro shall be a commercially reasonable substitute and substantial equivalent that may be either (i) used in determining the value of the ECU or (ii) tendered, in each case at the conversion rate specified in, and otherwise calculated in accordance with, the regulations adopted by the Council of the European Union.
     (c) Performance of any of the obligations described in subsections (a) or (b) of this Section may be made in the currency or currencies originally designated in the contract, security, or instrument, (so long as that currency or those currencies remain legal tender), or in the euro, but not in any other currency, whether or not the other currency (i) has been substituted or replaced by the euro or (ii) is a currency that is considered a denomination of the euro and has a fixed conversion rate with respect to the euro.