The comptroller by rule shall allow extra travel time to be claimed for computation of reimbursable travel expenses if a state employee obtains an airfare that is lower than the fare that would have been paid for travel for the same purpose but for a shorter period pursuant to this chapter, the result is a cost saving to the state, and the employee's additional absence is determined by the employing agency not to be detrimental.
Tex. Gov't. Code § 660.141