What you call residue may look to the police like a "useable amount." The police usually win this argument but you should have a lawyer to put forth your defense aggressively.
A useable amount may have been in the baggy. Regardless, it is time to retain an attorney and allow him or her to assist you.
Answer is for informational purposes only and does not create an attorney-client relationship.
If they can test it that is enough to charge. Not sure how they prove the felony. Get a lawyer and don't talk to the cops.
Most lawyers will give you a free consultation if you ask for one. We do this all the time, but remember that our time is valuable and you should stay with the people who help you the most.