Tex. Finance Code § 345.081

(a) A retail seller who has entered into a retail installment transaction under a retail installment contract to perform services or install goods for the modernization, rehabilitation, repair, alteration, improvement, or construction of improvements on real property shall obtain a certificate of completion or certificate of satisfaction signed by the retail buyer when all of the services have been performed or goods have been installed as required under the contract. A certificate is required regardless of whether a guaranty or warranty of the services or goods remains in force.

(b) A certificate of completion or certificate of satisfaction must be a separate writing and must have at the top in at least 10-point type that is bold-faced, capitalized, or underlined or otherwise conspicuously set out from the surrounding written material:

(c) The retail seller shall keep the signed certificate or a copy of the signed certificate until the second anniversary of the date of the certificate's execution.

(d) If performance of the services or installation of the goods required by the retail installment contract is not complete, a retail seller may not knowingly:

(1) induce a retail buyer to sign a certificate; or

(2) take or accept from the retail buyer an executed certificate.

(e) Execution of a certificate by the retail buyer is not a waiver of any guaranty or warranty made by the retail seller or a manufacturer or supplier.

(f) A retail buyer's failure or refusal to execute a certificate, without good cause, does not affect the validity of the retail installment contract.