If you were selling drugs, I don't think there's much else you can do here. I would assume you can either cooperate with the police or face the legal system on your own. It appears the cops are cutting you a break here by giving you the option of helping them to get rid of or reduce your charges. Yes, they are technically using you, but they also haven't arrested you and charged you with selling marijuana.
I'm not a lawyer and I won't try to tell you what you should do with your life. If it were me, I would work with the police and do all I can to keep my record clean. Perhaps this is a case of the police using you to try and move up the chain and catch a much bigger dealer. I would advise you to speak with a lawyer in your area and discuss the details of what the police are asking you to do to ensure you're not digging yourself into a bigger hole. I hope this helps.