There seem to be two things going on here, if I understand your narrative. One is a dispute over $500 and another is the apparent forging of your signature. What I would do is speak with the manager and tell him/her that they need to go back to the original deal, without the fake signatures because forgery is a bad thing, and they are risking their dealer license and jail. Say this VERY calmly and QUIETLY. Ask that they destroy the false agreement by shredding it in your presence, and then have you sign a new one that has the correct terms.
If they refuse, call the MD State Police Licensing Division and say you want to make a complaint against this dealer—let the MSP carry the ball.
Licensed in Maryland with offices in Maryland and Oregon. Information here is general, does not create a lawyer-client relationship, and is not a substitute for consulting with an experienced attorney on the specifics of your situation.
Whatever you do, just make sure you do not threaten filing a criminal complaint for the purpose of furthering your civil matter. That is blackmail. Decide your plan of action, and act accordingly. If they make your deal right fine, if they don't fine, but don't blackmail them.