Marijuana Possession: Obligated to help catch dealer?

I was riding with someone who had 2.5 grams of marijuana on them. He was stopping by a gas station to meet with someone and he was set up. The narcotics unit met us there instead of the guy he was supposed to sell to. After going downtown and working out a deal, giving them a bigger fish to go after (snitching instead of going to jail), the head of the unit says, "Yall call me Monday to help catch this guy and set him up". So basically, he wants us to call the guy, set up a place to meet, and instead of us being there, the cops are. Do I have to help them?