Tex. Gov't Code § 660.018

A state employee may not accept a reimbursement or advance for a travel expense from more than one source. A state employee who anticipates receiving or actually receives a reimbursement or advance for a travel expense from a person other than a state agency may seek an advance or reimbursement for the expense from a state agency only to the extent that the amount of the employee's advance or reimbursement from the other source is less than the amount of the total expense incurred.