Actually at the time Mahmoud did it the NBA had no fine for not standing during the national anthem.
The league later put the fine in after Mahmoud didn't stand.
The rule also has a lot of vagueness to it because Brandon Roy was not standing for the anthem and he would stay in the tunnel during the anthem one season (was not fined as well) for like a year until some punk ass ESPN writer found out and did a piece on it.
The league suspended Mahmoud and fined him. He also stated that the league didn't even fine him and the shyt was PR on the league's part to show the fans and media WHO was in charge.
Mahmoud said he was really thinking about quiting and saying the hell with it. But one of his mentors was like that is stupid because folks need to see you take a stand it is that important. So his mentor was like just go out there and pray during the anthem instead. You can reach a middle ground with the league and still protest during the anthem.