Tex. Tax Code § 321.507

(a) In each year in which a municipality imposes an additional municipal sales and use tax, if the revenue from the collection of the additional tax exceeds the amount of taxes computed for the municipality under Section 26.04(c), except for the amount required to be deposited in a special account under Subsection (b), the excess shall be deposited in an account to be called the municipal sales tax debt service fund. Revenue deposited in the municipal sales tax debt service fund may be spent only for the reduction of lawful debts of the municipality, except that deposits that exceed the amount of revenue needed to pay the debt service needs of the municipality in the current year may be used for any municipal purpose consistent with the municipal budget.

(b) Revenue from the collection of the additional municipal sales and use tax in each of the first three years in which the tax is imposed in the municipality in excess of the amount determined as provided by Section 26.041(d), for each year shall be deposited in an account to be called the excess sales tax revenue fund. During those three years, revenue deposited in the excess sales tax revenue fund may be spent only if and to the extent that taxes or other revenues of the municipality are collected in amounts less than anticipated. After that period, the revenue in the fund may be used for any municipal purpose consistent with the municipality's budget. The fund ceases to exist when all revenue deposited in the fund has been spent. This subsection does not apply to a municipality that does not impose a property tax.