What is the ruling on taking a percentage of the survey amount agreed upon with researchers, given the existence of doubts about the credibility of some forms, and after the researchers have received their dues and signed for receiving a higher amount than what they actually collected?

You must return the surplus you took from the office, because you are merely an agent for them in contracting with researchers, and you must return the difference unless the officials authorize you to dispose of it. As for the negligence of some researchers in their research, their sin is upon them, and no sin falls upon you unless you cooperate with them in it. The researcher who did not complete his research takes from the wage according to what he has done, and it is not permissible for you to take anything from the researchers' wages.