Product: Debt collection

I entered into an agreement with Portfolio Recovery Associates LLC last year, XX/XX/2019 and committed to pay {$200.00} every month until my credit card debt was paid in full. I have faithfully mailed in a check every month since the agreement was made and have never had a problem. Until now. Last week I received a phone call telling me I had submitted a payment in the amount of only {$160.00} and not the {$200.00} as I had previously agreed upon. I informed the woman who called me that I had submitted my payment in the amount of {$200.00} and I had my check from the bank backing up this claim. She asked me to submit a copy of that check ( which I went to my bank and got a copy of ) along with a letter telling them what the problem is with the payment amount. I did as requested and also stated in that letter that I was disputing that claim that they had made. I sent the letter and a copy of that check to Portfolio Recovery Associates, LLC XXXX XXXX XXXX -Back Office XXXX XXXX VA XXXX on the Monday of this past week. I just received another phone from them telling me I was in violation of the agreement and now the agreement was null and void since I had not submitted the full {$200.00} as agreed upon. I argued and argued with the man and tried to explain to him that this was a bogus claim and that I had proof for what the amount of XXXX 's payment was. He then asked me to email them a copy of that copy of the check from my bank to him and I did so. He then said I would have to enter into another agreement since I had VIOLATED the first one. It was nothing but a runaround over and over with him. When I threatened to call XXXX XXXX XXXX he put me on hold and contacted his supervisor and then came back to me and said they would " knock off '' the {$30.00} I was short on the XXXX payment! I was NOT short on the XXXX payment and I have a copy of that check to prove that. I got no where with this man or his supervisor. I was told I could start sending in payments of {$200.00} a month in a new agreement. He then tried to get me to tell him on what date I would submit my payment from here on out and I told him I would submit that payment as I have always done so that it would be in their hands on the first day of every month as I had originally agreed upon. He then asked me to verify the last 4 digits of my social security number and I refused, l informed him I do not give out that kind of personal information over the phone and he then proceeded to tell me good bye and to have a nice day. I'm not sure what kind of stunt they are trying to pull with me at this point and I no longer trust them when I submit a payment to them. I also informed him the XXXX payment I had sent in was cashed a week earlier than they have ever cashed it before ( it was not a problem for me that they had done so but I did notice it was cashed earlier than they normally did ) and that someone had written a number down in the memo part of my check a number that did not make any sense to me and the last part of the numbers you can not make out. I did NOT write those on there and I do not who did. My frustration level with these people is over the top, you can not discuss the issue involved with them and they do nothing but give you a run around. I was also informed on the first phone call ( last week ) that since I had not submitted the full amount of my payment that the agreement I had with them was now null and void. I don't know what they are going to do now. I still owe them {$6000.00} according to the man I talked to today. I also asked him not to call me on the phone anymore and to only contact me through the U S mail. He ignored what I had to say. What recourse do I have now? Will my payments still be accepted? Will they bring formal charges against me when I am innocent of what they are claiming and have proof to show this? I don't know what to do at this point and I no longer trust them. Thank you for any help you can give me. 
Sincerely, XXXX XXXX