Does sin befall one who paid a sum to the broker who brought the project, without knowing his identity or whether he works on the same project?

There is no sin upon you for delivering the amount to the project applicant who brought it, for since the intermediary authorized him to do so, your responsibility is discharged, because the hand of an agent is like the hand of his principal. And even if the intermediary did not authorize him to do so, there is still no sin upon you.