If the "residue" is drug residue, you can potentially be arrested and charged with drup possession: this is your wallet, and there is a strong presumption that any residue in it was yours. It will not be very persuasive or credible to say that you did not know there was an empty baggie in the wallet, since when someone gets a wallet, they almost invariably look through it, both to see what is in it and as they put their own money, credit cards, etc. into it. You are strongly advised to speak with a criminal defense attorney and to *not* talk to the police until you do.