What are my chances of winning a decision, if a debt collector is trying to collect $899 + fees but $647 was assessed in error ?

Unlike Atalantic City, no quality attorney will lay odds on a court case outcome

This is assuming there is no court case yet. Send a written dispute to the debt collector with a copy of all of your evidence. If they are reporting the higher balance to the credit bureuas, send disputes to them as well. If they continue to go after the full amount after receiving your disputes, contact an attorney who specializes in consumer law to see if you have a claim under the Fair Debt Collection Practices Act or the Fair Credit Reporting Act. Most will provide a free consultation and take good FDCPA cases on a contingency basis. If they are already suing you, I recommend contacting a consumer attorney right away. If you can't find an attorney, bring all of your evidence to the court and politely present it when the case goes to trial.

I concur with the prior posters. It is never a good idea to give someone a representation on what their "chances" are, because there are too many unknowns. Contact the creditor with your evidence in writing in a respectful and professional manner and do whatever negotiation you can. Given the small amount in dispute, I doubt you will find an attorney to take this case because it would cost more to defend than it is worth.