Tex. Gov't Code § 805.003

The cost of reinstating or purchasing service credit under Section 805.002 is determined according to the statutes that govern the reinstatement or purchase of the type of service credit in the system in which it is to be reinstated or purchased. All payments for service credit reinstated or purchased under Section 805.002 must be made before retirement or the first payment of a death benefit annuity, as applicable, or before a later date if allowed for members of the retirement system in which the credit is to be reinstated or purchased.