(a) A commission is payable at the time provided in the commission agreement. If payment of the commission is conditioned on the occurrence of an event and that event does not occur, the person obligated to pay the commission is not required to pay the commission.
(b) Except as provided by Subsection (c), a commission is earned on the earlier of the date that:
(1) an event occurs that, under the commission agreement, defines when the commission is earned; or
(2) the person obligated to pay the commission enters into a purchase contract or a lease during the period prescribed by the commission agreement for all or part of the commercial real estate if the purchase contract or lease is contemplated by the commission agreement and if the parties to the purchase contract or lease are contemplated by the commission agreement.
(c) If a broker has earned a commission under a commission agreement relating to a lease transaction and the commission agreement provides that the broker may receive an additional commission when the lease is modified to expand the lease space or renewed, the additional commission is earned when:
(1) the broker performs all the additional services relating to the lease modification or renewal expressly prescribed by the commission agreement; or
(2) the broker first earned a commission under the commission agreement if the commission agreement does not expressly require the broker to perform additional services relating to a lease modification or renewal.