Tex. Transportation Code § 366.174

(a) An authority may maintain a revolving fund to be held in trust by a banking institution chosen by the authority separate from any other funds and administered by the authority's board.

(b) An authority may transfer into its revolving fund money from any permissible source, including:

(1) money from a turnpike project if the transfer does not diminish the money available for the project or the system, if any, of which it is a part to less than an amount required to be retained by the bond proceedings pertaining to the project or system;

(2) money received by the authority from any source and not otherwise committed, including money from the transfer of a turnpike project or system or sale of authority assets;

(3) money received from the state highway fund; and

(4) contributions, loans, grants, or assistance from the United States, another state, a political subdivision of this state, a foreign governmental entity, including the United Mexican States or a state of the United Mexican States, a local governmental entity, any private enterprise, or any person.

(c) The authority may use money in the revolving fund to:

(1) finance the acquisition, construction, maintenance, or operation of a turnpike project or system, including the extension, expansion, or improvement of a project or system;

(2) provide matching money required in connection with any federal, state, local, or private aid, grant, or other funding, including aid or funding by or with public-private partnerships;

(3) provide credit enhancement either directly or indirectly for bonds issued to acquire, construct, extend, expand, or improve a turnpike project or system;

(4) provide security for or payment of future or existing debt for the design, acquisition, construction, operation, maintenance, extension, expansion, or improvement of a turnpike project or system;

(5) borrow money and issue promissory notes or other indebtedness payable out of the revolving fund for any purpose authorized by this chapter; and

(6) provide for any other reasonable purpose that assists in the financing of an authority as authorized by this chapter.

(d) Money spent or advanced from the revolving fund for a turnpike project or system must be reimbursed from the money of that turnpike project or system, and there must be a reasonable expectation of such repayment at the time of authorization.