Am I entitled to a percentage of the profits from the sale of a plot of land that I purchased for a charity I worked with, given that I made efforts to increase its market value, and knowing that my primary role was managing the da'wah and charitable aspects, not the investment aspect?

If you request money in exchange for benefiting the association, as a donation from them, then there is no objection and it is recommended for them to reward you. However, if you make this a condition for selling the land, then if selling real estate is part of your core job for which you receive a salary, it is not permissible to take a percentage. Otherwise, it is permissible to stipulate a commission as a brokerage fee, provided that it is outside official working hours; because working hours belong to the institution, and the contract or custom determines whether this is part of your core job or not.