When cops offer someone a deal on the spot, are they lying?

From what I understand, only a prosecutor can make deals.. I've been watching this show "DEA" on Spike all day, and they always get people to snitch on their higher up and they always offer some vague improvement of their lot. So does this cooperation actually help these guys, and if they offer a specific deal and the guy cooperates, will they normally get it? Just wondering if these DEA guys are lying