The officer is offering you a choice. if you choose not to cooperate, you will be going to Court on the charges of possession. If it is your first offense, you can keep it off you record. If you cooperate with them, they can agree to not bring charges against you or drop charges already filed. This happens everday in every part of the State. Visit www.kliszlaw.com to discuss. Tim Klisz
This is a common law enforcement technique. When I was a state and federal prosecutor, we utilized cooperating defendants on a regular basis. It is important to get this deal in writing. While it may be okay to rely on this particular officer's statement to you that your small possession case will go away, a safer way to go is to get an attorney that can negotiate and properly memorialize a deal in writing. You should also fully understand the potential consequences of not cooperating so you can evaluate whether the "risk" is worth it. There are several good defense lawyers in your area, and assuming things are as straight forward as they seem, it shouldn't cost you too much money.
Unfortunately, police officers have broad discretion to conduct a traffic stop. In your case, tinted windows can be a reason to execute a stop. However, they may not be unless they are in violation of the Michigan Vehicle Code. You should proceed with caution when it comes to dealing with undercover drug enforcement teams because there will be no "contractual" obligation to dismiss the charges or negotiate the charges against you. However, usually drug enforcement teams honor oral argeements. On the other hand, they play by their rules and not the court's or the prosecution's: neither branch is governed by what law enforcement tells you.
You should contact a lawyer. Do not be penny wise and pound foolish.