Tex. Tax Code § 152.086

(a) The taxes imposed by this chapter do not apply to the sale or use of a motor vehicle that:

(1) has been or will be modified before the second anniversary of the date of purchase for operation by, or for the transportation of, an orthopedically handicapped person; and

(2) is driven by or used for the transportation of an orthopedically handicapped person.

(b) The comptroller shall promulgate rules to ensure that motor vehicles exempted from taxation by this section are used primarily by orthopedically handicapped persons. The comptroller may require any individual seeking exemption under this section to present information establishing qualification for the exemption.

(b-1) The seller of a motor vehicle may not collect the tax from the purchaser of the motor vehicle if the purchaser:

(1) signs at the time of the purchase an exemption certificate that:

(A) is on a form designated by the comptroller; and

(B) contains all information the comptroller considers reasonable to establish qualification for the exemption at the time of sale; and

(2) presents any other documentation or information the comptroller requires by rule.

(b-2) Notwithstanding any other provision of this section or other law, the seller of a motor vehicle may rely on a properly executed and signed exemption certificate under Subsection (b-1) and does not have a duty to investigate the propriety of an exemption certificate that is valid on the certificate's face. A seller who relies on an exemption certificate as provided by this subsection is not liable for the payment of motor vehicle sales taxes that would otherwise be due as a result of a motor vehicle sale.

(c) If the comptroller finds that the motor vehicle is not used primarily for the purposes specified in this Act or that the exemption should not have been granted, the comptroller shall assess the tax in an amount that would have been due had the exemption not been given under this section.