question about a misdemeanor

I had gotten in some trouble about 7 or 8 years ago, before I knew about any of this information. I was ignorant and plead guilty to a misdemeanor charge. I was sentanced to house arrest and probation. It was a long time ago but I remember paying my what I thought was the full amount of fines, completing my house arrest and probation and then I figured it was over. Well a little under a year ago, I received a letter stating I stilled owed 1900 federal reserve notes. I was still ignorant to this information at that time, so I didn't know how to fight it. I wasn't sure how they could let me off probabtion without satisfying my fines and then years later, demand more debt instruments from me under the threat of a warrant for my arrest being issued. I grudingly agreed to pay as little as they would let me, which is $20 per month until the debt was taken care of. I signed nothing , but I guess I made a verbal contract. Since then I have become a Secured Party/creditor. What are my options to take care of this situation with those corrupt entities responsible for extorting more from me. I am trying to figure out the best way for me to handle this situation. I would appreciate any advice anyone might have for me.