Tex. Business & Commerce Code § 661.005

(a) If a subject or medium of payment of a contract, security, or instrument is the European currency unit or a currency that has been substituted or replaced by the euro, the euro is a commercially reasonable substitute and substantial equivalent that may be:

(1) used in determining the value of the European currency unit or currency, as appropriate; or

(2) tendered, in each case, at the conversion rate specified in, and otherwise computed in accordance with, the regulations adopted by the Council of the European Union.

(b) A person may perform any obligation described by Subsection (a) in euros or in the currency or currencies originally designated in the contract, security, or instrument if that currency or those currencies remain legal tender, but the person may not perform the obligation in any other currency, regardless of whether that other currency:

(1) has been substituted or replaced by the euro; or

(2) is considered a denomination of the euro and has a fixed conversion rate with respect to the euro.

(c) The following occurrences are not considered a discharge of, do not excuse performance under, and do not give a party the right to unilaterally alter or terminate a contract, security, or instrument:

(1) the introduction of the euro;

(2) the tender of euros in connection with any obligation described by Subsection (a);

(3) the determination of the value of any obligation described by Subsection (a); or

(4) the computation or determination of the subject or medium of payment of a contract, security, or instrument with reference to an interest rate or any other basis that has been substituted or replaced because of the introduction of the euro and that is a commercially reasonable substitute and substantial equivalent.