Is what the expert did permissible in Islamic law—namely, deception and demanding a percentage of the profits after having stated that he wanted nothing—and is the wronged party sinful if he does not give him what he asks for, or gives him only a portion of it?

If a person explicitly states that they do not want compensation for managing the portfolio and that they are donating their work, then they have no legal right over you. However, it is an act of kindness to give them something that pleases them if their effort bears fruit and your money is recovered, in accordance with the Almighty's saying: "Is the reward for goodness anything but goodness?"