My name is XXXXX XXXXX I'd be happy to answer your questions today.
If the warrant is backed by probable cause
to believe that they will find marijuana in the apartment, they're allowed to come in and look around, even if the detective subjectively is hoping to find other drugs. The warrant will let him in the door. Once he's there, he's free to look anywhere that marijuana might be found - which is also going to also any place that meth might be found. Drugs are small, so a warrant for drugs lets them look just about anywhere.
It actually does make sense - they heard you might have marijuana, so they got a warrant to go in and look around. They found meth, which is a much more dangerous drug. They didn't find enough to charge you with selling meth, so they assume you're the end user, and they want to use you to go after the guy who IS selling meth. If you're willing to cooperate, they'll make a deal with you, and they won't charge you with distribution of marijuna - which the evidence supports. They don't have to make a deal with you, and you don't have to agree to go along with it. They may not be investigating someone close to you - it sounds like they just decided that they are willing to let you go in order to get the bigger fish (and the person who poses more of a danger to society).
If you do agree to help them get the meth dealer, make sure there is something in writing that says that you won't be charged for possession of the meth or distribution of marijuna - you want to be clear on exactly what you're getting into. You don't want to go along with everything and find yourself STILL charged with distribution of marijuana.