Citibank Credit Card Complaint

Citibank Credit card department,

Billing disputes California

On -/-/2016 Costco changed from - to Costco Anywhere Citi Visa. On -/-/2016 I paid $5900.00 to -. On the first statement from Citi Visa I received a bill for $5900.00 the amount that I had paid - ; Citi informed me that I had not paid - and therefore ; I needed to pay them. I informed them that I had payed - and sent them a copy of my bank statement that showed the $5900.00 being deducted and payed to -. Citi Visa refuses to correct their mistake and issue credit to me for $5900.00. Because Citi Visa has not corrected their mistake my credit score has been down graded and when I tryed to refinance my Mortgage I was told that I was going to pay $250.00 more because Citi Visa had reported that I was late with payments. Customer Service at Citi Visa has told me on different occasions that they had found the payment only to receive letters later informing me that I still owe the $5900.00 because they had not found the payment after all. I have been trying to get this issue resolved for over 9 months.