How are credit card companies allowed to collect on accounts?

I have an account that was sent to an attorney a couple of years ago and I made a payment agreement with them and have been paying them every month. During the time I have been paying them, the credit card company has still been sending me bills on the account and charging me interest on their web site as well as what I am paying through the Lawyers web site. Now after 2 years, they say they are pulling it and are going to sent it to another company when I have it paid to 800.00 according to the office it is in now and was going to pay off in October, but because of them still charging interest the way they did, they show me owing 1700.00 to send to another company. I can get verification from the law firm/collection of all my payment that show what my amount should be because I

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