Citibank Debt Collection Complaint

Citibank Debt collection department,

Credit card False statements or representation Attempted to collect wrong amount Massachusetts

I am about to retire. I went in to my credit report to make sure everything was OK. I found an item for a company credit card from years ago. Over the years I had paid much more on interest than I originally used on the card. The amount on the credit card was for $20000.00. I called the company on the credit report, ( - - - ) which is a collection company. We discussed the matter over the phone and came to an agreement to satisfy this matter for the price of $5000.00. Check # - from -, -, MA I sent them the money via a check over the Internet. They in turn were supposed to send me an email confirming this arrangement and that this amount was to satisfy the matter completely. This was two days ago and I have yet to receive a message confirming this arrangement. At this point I am leery that the company has simply cashed my check and taken no further action to resolve this issue on my credit report.