This all sounds like a bust that went forward. Your brother may still be charged. He signed an agreement, presumably without advise of counsel Bad idea. So, have your brother hire a lawyer and have your brother reveal all the details of the arrest, bust, and letter agreement he signed. Only then can this get pieced together.
The cops can do this and they do it all the time. If your brother gives up whatever he agreed to give up (usually an amount and specific type of drugs), then they will not file the charges (hopefully. It is much better to enter these kind of agreements with the consent of the prosecutor and with your lawyer there to ensure fairness.) If he does not live up to his end of the agreement, they can file the charges. They do not have to bring the charges immediately but must bring them within the statute of limitations which is years long.
The statue of limitation on criminal offenses in Illinois is three years and he can be charged at any time within that time span. As for his written agreement with the police, be advised that he is being used and the agreement will not serve as a defense if he is charged. Such agreemnets need to be made with the state's attorney in the jurisdiction where the offense occurred (Will County in his case). The police can, and often do, breach such agreements. He should immediately contact an expereinced criminal defense attorney.
They are well within the law in terms of the time to file the charges. Hopefully your brother has a written deal with them to ensure a specific outcome and he isn't just on the hook to the police.
He should contact a lawyer about this.