Product: Money transfer, virtual currency, or money service

I am writing you in regard to a bill that is claimed to be outstanding. 

For your information, I have a high deductible health plan ( HDHP ) and a health savings account ( HSA ) with Optumbank. I received a bill from XXXX XXXX for {$340.00} sometime in XX/XX/XXXX. I paid the bill on XX/XX/XXXX, using Optumbank. The payment confirmation number is XXXX. 

Around XX/XX/XXXX, I received another bill from XXXX XXXX for the same amount. I contacted XXXX customer service sometime in XX/XX/XXXX. I informed them I already paid. They claimed they did not receive payment. 

Around this same time in XX/XX/XXXX, I reviewed my Optumbank account and noted that the money {$340.00} left my Optumbank account bank in XX/XX/XXXX. I contacted Optumbank. Optumbank confirmed the payment went out to XXXX XXXX and that the payment was processed. The money in the amount of {$340.00} is no longer in my Optum bank account. I obtained a copy of the payment confirmation via email. 

In XX/XX/XXXX, I provided XXXX XXXX billing department with a copy of the confirmation of payment by mail. This included the transaction date, transaction amount of {$340.00}, and the transaction confirmation number of XXXX. This also included a contact number for Optumbank for Optumbank to provide more information for locating payment at XXXX. 

In the end of XX/XX/XXXX, I received another bill from XXXX XXXX claiming that I still owe the same amount of {$340.00} for the same service date. 

On XX/XX/XXXX, I once again sent proof of payment to XXXX XXXX XXXX Department. This includes the transaction date, transaction amount of {$340.00}, and the transaction confirmation number of XXXX. This also included a contact number for Optumbank for Optumbank to provide more information for locating payment at XXXX. 
On XX/XX/XXXX, I contacted Optum Bank again. They once again confirmed that I paid XXXX XXXX {$340.00} on XX/XX/XXXX. They sent me confirmation of payment via email. This includes the transaction date, transaction amount of {$340.00}, and the transaction confirmation number of XXXX. This also included a contact number for Optumbank for Optumbank to provide more information for locating payment at XXXX. I asked them if there was anything else they could give me to show proof of payment. They said this confirmation information was all they could give me. 
On or about XX/XX/XXXX, I received an email confirmation of the same payment. This includes the transaction date, transaction amount of {$340.00}, and the transaction confirmation number of XXXX. This also included a contact number for Optumbank for Optumbank to provide more information for locating payment at XXXX. 
On XX/XX/XXXX, I am sent the same payment confirmation that was provided by Optum Bank yet again. This includes the transaction date, transaction amount of {$340.00}, and the transaction confirmation number of XXXX. Please find a copy of the payment confirmation attached to this letter. 
On XX/XX/XXXX, at approximately XXXX am Eastern time, I called XXXX XXXX XXXX Department. I informed XXXX XXXX XXXX Department that I paid the alleged outstanding bill of {$340.00} back in XX/XX/XXXX. I told them I also sent XXXX letters containing the payment confirmation from Optumbank. They hung up on me. 
At this point I do not know which entity is at fault. 
Optumbank claims they transferred the money and deducted from my account. Optumbank claims the only proof they can give me of payment is the information they provided by email with the transaction date, transaction amount and confirmation. 
XXXX XXXX XXXX Department claims I never paid the bill and that I never contacted them. However, I contacted them twice by phone and now three times by letter showing proof of payment.

I think I have exhausted all attempts at administrative remedies and no further phone calls or showing of proof of payment will make a difference. All I know is I am out of {$340.00} from my XXXX account and now XXXX XXXX is threatening to send my alleged unpaid bill to collections so it can affect my credit history. 
Someone has {$340.00} of my money, and I am without wherewithal to find where my money went. 
Please help. I think I have done everything I can to rectify this situation but neither Optumbank nor XXXX XXXX XXXX Department are willing to help me. I dont know what to do anymore. 
Thank you