Product: Debt collection

I have received three bills from Aargon. Dated XXXX, XXXX, XXXX. 
The original debt is for {$200.00}. I did n't dispute that debt. I have written a three letters in response to each letter. 

The first letter dated XXXX XXXX was to pay in full with payments every two of {$10.00} and in exchange Aargon would cease collections on any existing accounts, and show me in good standing on this account as long as I continued payments on the account. Shortly after the letter was received all Aargon related accounts were no longer reported to the credit reporting agencies. This was an act of agreement to the terms. They received their first payment on XXXX XXXX, XXXX as an act of good faith previous to the terms being accepted. The terms of the agreement also stated if at any time Aargon were to demand full payment, the agreement would no longer be valid, considered null and Aargon would cease and desist future collections and reporting on this and accounts regarding XXXX XXXX XXXX currently. Accepting a payment would be considered an acceptance of payment. Aargon accepted the payment the first payment. At the end of payments, Aargon would send a statement, paid in full. 

On XXXX XXXX, I received a letter to the one I received today stating a stating there was no payment arrangements made and demanding total amount due. It also demanded a larger balance. It stated I owed {$190.00} when I owed {$180.00}. As a result, I sent a letter dated XXXX XXXX, XXXX offering Aargon two options for payments to continue paying on this balance. I offered for Aargon to make the final decision on which of the two offers I am making to choose from. The original balance is {$200.00}. Due to demand for a higher balance than what is owed and denial of the payment arrangements, I was willing to pay 50-percent of the debt, {$100.00}. I was willing to continue on the current {$10.00} every two-weeks payment arrangement. Or if they reported it to the credit reporting bureaus that I am in good standing and paying on time, like a line of credit, I was willing to pay {$130.00}. As well, I was willing to continue on the currently {$10.00} every two-weeks payment arrangements. 
The states the following : Amount owed : {$190.00} Interest : {$0.00} Charges/Fees {$0.00} Your Payments {$10.00} Total Balance Due {$190.00} The balance should be {$180.00} Today, I receive a letter dated XXXX XXXX stating there is no payment arrangements and demanded full payment. They have accepted three payments to date.This brings the balance to {$170.00}. As a part of the terms and conditions in the second letter I sent them, I stated if I did not receive a correspondence from them by depositing a payment it was an acceptance of the terms and conditions. The balance was stated as the following : Amount owed : {$180.00} Interest Accured 0 Charges/Fees 0 Your Payments -20 Total Balance XXXX The balance should be {$170.00}. 

They refuse to acknowledge accepting the payment arrangements and are reporting I own more than I do on my statements I am receiving. 

In the third and final letter I have sent out today, I have sent a cease and desist, along with informing them I will not be sending out any more payments as they have failed to honor the terms and conditions of the payments arrangements they accepted when accepted the second payment. I sent a first payment in good faith.