Product: Credit card or prepaid card

I made payment to Barclay Card US on XX/XX/XXXX, in the amount of {$9700.00} via check number XXXX, despite my balance per my statement due only being {$920.00} and being due XX/XX/XXXX. The overpayment was because I had made purchases in XXXX that totaled the much larger amount, and I always pay in advance as I don't like revolving credit. The check cleared my bank on XX/XX/XXXX, and clearly references " Barclays XXXX '' on the back as the one that cashed it. However, it never appeared on my account that I check regularly on-line. I started calling Barclay 's on XX/XX/XXXX to inquire about it, and they claimed they had no record of the payment, so I provided them the check number and amount numerous times over the phone, as presumably that should be enough data to easily track it down. Realistically, how many checks could they have received with check number XXXX in the amount of {$9700.00} between XX/XX/XXXX and XX/XX/XXXX? They opened up case number XXXX. Regardless, by XX/XX/XXXX, they still hadn't resolved it, so I asked their employee number XXXX for a fax number to fax a copy of the front and back of the cashed check to, and I faxed it to them at the number he provided at XXXX. Despite him being on the phone with me while I faxed it, he was unable to verify that it reached their fax machine and reportedly unable to speak to anyone in that department, though I have the Transmission verification report showing it went through. I asked for an email address to send it to instead, but he wouldn't give me one. I asked for a fax number wherever he was in their offices so he could get the document directly and verify it was recieved, but he wouldn't give me one. So, I asked to escalate the call, and he said someone would call me back. I received a callback from someone on Sunday, XX/XX/XXXX. She said she couldn't find any document or reference to any faxed document that I sent them on the XX/XX/XXXX, so she ended up giving me a different number to fax it to, XXXX, as well as an email address XXXX, to send the copy of the check. I did so from my office the next business day, Monday, XX/XX/XXXX, and have the transmission verification report as well as the sent email to show it went through. However, when I have called them every day this week they couldn't verify whether or not they received this information again. They finally admitted today ( employees XXXX # XXXX and XXXX # XXXX ) that they did receive the documentation, but they claimed they hadn't received it until XX/XX/XXXX. Not possible as a fax or email are both maybe a second or two delay, but also not surprising based on what I've experience the past three weeks with this company. So, I asked to speak to a supervisor, but employee XXXX refused to connect me with one and said nobody was available. I asked for a timeframe that this would resolved, and they refused to give me one. I  informed them that I've been getting daily emails from Barclay Card US indicating that I'm delinquent on my payment and asked them to do whatever they needed to do  in their system to get those to stop as they had acknowledged that they had proof that this was their mistake, but they indicated it was automated and they couldn't stop them. I informed them that this was obviously a problem then, because their reporting delinquencies to credit bureaus and reporting account statement balances was likely automated as well, so they were thus likely to falsely report negative issues that may affect my clean/excellent XXXX credit rating. They had no response for me. 

I need some help please. Barclay Bank should be required to promptly make corrections to their errors in a reasonable timeframe to ensure that their incompetence does not erroneously harm the financial standing of their consumers.