First of all you can get the case thrown out because they didn't read you your rights before you were processed and so forth. Secondly any judge in their right mind would look at this and throw it out simply because your story is provable. Passing off the money when you clearly didn't know it was fake is not illegal BUT they will end up taking that money from you :(

You will not have to pay for an attorney SINCE you can legally get one for free :) Any prosecutor with common sense would see the err in their ways and let you go with a technicality (no rights being read violating your due process).