Is it permissible for a partner in a land trade to purchase a plot of land that has been agreed upon to be sold to another person at a certain price, and then sell it to the same person at a higher price, and is it permissible to take the stipulated commission after the sale is completed?

It is not permissible for a real estate agent who has been entrusted with selling land to buy it for himself, unless it is at the highest price it has reached. This is because they are trustees and agents for the landowners, and they must fear Allah and be truthful in securing the best prices for them.