To follow is a pasted a copy of the letter that I sent to Citibank. 
CitiMortage Customer Research TeamXXXX XXXX, MD XXXXDear Madam/Sir, Our CitiMortgage payoff statement quoted per diem interest of {$84.00} per day. 
CitiMortgage charged at the per diem rate of $ XXXXCitiMortgage charged us interest at this rate through XX/XX/2015. 
The mortgage was paid in full electronically on XX/XX/2015. 
The total CitiMortgage per diem interest charge was $ XXXXThe correct charge is {$2300.00}. 
Please promptly refund the remaining {$200.00} due and mail it to our new address listed above. 

I doubt that this is the first time Citi has issued an incorrect payoff that yields them an extra {$200.00} that they have not earned. I worry that there may be thousands of citizens who have been taken advantage of in this way. Most consumers do not realize that banking in our current post XXXX means that every number and every fee must be checked. Please look into this and let me know how I can help. 
