Tex. Educ. Code § 25.038

The receiving school district may charge a tuition fee to the extent that the district's actual expenditure per student in average daily attendance, as determined by its board of trustees, exceeds the sum the district benefits from state aid sources as provided by Section 25.037. However, unless a tuition fee is prescribed and set out in a transfer agreement before its execution by the parties, an increase in tuition charge may not be made for the year of that transfer that exceeds the tuition charge, if any, of the preceding school year.