My practice is in Georgia and I am a workers' compensation attorney. In general , a simple client letter discharging my services is enough to discharge my services. It appears that you are in Michigan and do not practice law in your state and this response shall not be construed as professional legal advise under Michigan law.
Generally, I would refer to the engagement letter as a guide. In the engagement letter, it will generally state the terms and protocol should you and your attorney part ways. Also, I would follow up with the State Bar which has jurisdiction over your agreement.
Your question about the past money owed is a more sticky problem. Again, I would refer your engagement or retention letter as to the resolution of fees. Some agreements have an arbitration clause that allows the parties to engage in alternative dispute resolution (ADRR) methods. If you owe attorney's fees, there is a good chance that your lawyer will be able to recover unless he agrees to a lesser amount or you find resolution via a court order or ADR depending on your state's guidelines.