Is it permissible to take a percentage of money from a person who fills out scientific research surveys in exchange for a certain amount, knowing that I am an intermediary in obtaining the funding, and that he is satisfied with the amount he will receive?

If the entity funding the research allocates a sum for filling out questionnaires, then it is not permissible to dispose of it except according to their condition. It is also not permissible to deduct a portion of it without the knowledge of the friend who will perform the task.