Tex. Tax Code § 151.155

(a) Except as provided by Section 151.3181 for property used in manufacturing, if a purchaser certifies in writing to a seller that a taxable item sold, leased, or rented to the purchaser will be used in a manner or for a purpose that qualifies the sale of the item for an exemption from the taxes imposed by this chapter, and if the purchaser then uses the item in some other manner or for some other purpose, the purchaser is liable for the payment of the sales tax on the value of the taxable item for any period during which the item is used in the divergent manner or for the divergent purpose.

(b) The value of an item of tangible personal property is the fair market rental value of tangible personal property, which is the amount that a purchaser would pay on the open market to rent or lease the property for his use. The value of a taxable service is the fair market value of the taxable service, which is the amount that a purchaser would pay on the open market to obtain the service for the use of the purchaser.

(c) If an item of tangible personal property has no fair market rental value or if a taxable service has no fair market value, the original purchase price shall be the measure of tax.

(d) At any time, the person making the divergent use may cease paying tax on the fair market rental value or fair market value and may pay sales tax on the original purchase price without credit for taxes previously paid.

(e) A purchaser of a taxable item who gives an exemption certificate is not liable for the tax imposed by this chapter if he donates the taxable item to an organization exempted under Section 151.309 or 151.310(a)(1) or (2) of this code; except that any use by the purchaser of the taxable item other than retention, demonstration, or display shall be subject to taxes imposed by this section.