Alex and Aisha had a long discussion about payment regarding him helping her out of a tough spot. It wasn't that Alex actually gave a fuck about the money. In fact, 500,000 dollars was virtually pennies in his eyes when you considered the fact that he was raking in billions every year from the cartel.
Rather, it was the idea of a establishing a link between himself and Aisha, a contract if you will. As long as he had a reason to reasonably contact her, she would keep in touch, and he would be allowed to properly establish a relationship between the two of them.
Aisha didn't actually have a way to pay Alex anyway. She had no assets of her own, not even a bank account in her name. Everything was paid for by her father. But regardless of this fact, she still promised to pay Alex what he desired for helping her. Not that she even knew the actual worth of half a million dollars.