Tex. Tax Code § 151.424

(a) A taxpayer who prepays the taxpayer's tax liability on the basis of a reasonable estimate of the tax liability for a quarter in which a prepayment is made or for a month in which a prepayment is made may deduct and withhold 1.25 percent of the amount of the prepayment in addition to the amount permitted to be deducted and withheld under Section 151.423 of this code. A reasonable estimate of the tax liability must be at least 90 percent of the tax ultimately due or the amount of tax paid in the same quarter, or month, if a monthly prepayer, in the last preceding year. Failure to prepay a reasonable estimate of the tax will result in the loss of the entire prepayment discount.

(b) In order to qualify for the deduction permitted by Subsection (a) of this section, the taxpayer must make the tax prepayment:

(1) on or before the 15th day of the second month of the calendar quarter for which the prepayment is made if the taxpayer pays the tax quarterly; or

(2) on or before the 15th day of the month for which the prepayment is made if the taxpayer pays the tax monthly.

(c) A taxpayer who prepays the tax liability as permitted by this section must file a report when due as provided by this chapter. The amount of a prepayment made by a taxpayer under this section shall be credited against the amount of actual tax liability of the taxpayer as shown on the tax report of the taxpayer. If there is a tax liability owed by the taxpayer in excess of the prepayment credit, the taxpayer shall send to the comptroller the remaining tax liability at the time of filing the quarterly or monthly report. The taxpayer is entitled to the deduction permitted under Section 151.423 of this code on the amount of the remaining tax liability.

(d) If the amount of a prepayment exceeds the actual tax liability, the excess of the prepayment shall be credited against future tax liability of the taxpayer or refunded to the taxpayer as provided by Subchapter C of Chapter 111 of this code.