Can You Still Be Charged After Cooperating With the Police

My question involves criminal law for the state of: Alabama

Ok, so I was traveling from a different state w/ my gf through Alabama and was pulled over because of registration. The cop then proceeded to ask me if I had marijuana because he smelled it which I said no. After asking to search my car I said he needed to get a warrant but then later admitted to having 1 1/2 oz. I also had 10 pills of E which was later found. When I was taken back to the station I spoke w/ the Lt. and 2 NARC officers who offered me amnesty in return for information of a "bigger fish". I gave him tons of information about an operation that I used to be involved(I did not mention my involvement I said it was just good friend of mine) in return for my release, my gf's release, return some cash I had in the car, and a clear record. Now I have two parts to this question. They did not take any information from me/fingerprints/booking etc. I just walked out the back door an hour later and continued on. Will this come back to bite me in the ass because I didn't get it in writing? If so, what are my legal rights? The other part to the question is that I know my friend is going to get busted w/ this operation. If he were to mention some old "partners" even though I havn't been involved in 6 months can I get charged w/ conspiracy even though there is no evidence? Any help would be greatly appreciated

You committed a crime, were caught by the police and confessed, so of course it's possible for them to charge you. Odds are that if they made a deal with you and they are satisfied that you lived up to your end, they won't. In terms of "is it best to have the agreement in writing," yes, but you can't unring a bell.