Product: Debt collection

I'll provide as much information as possible to make my story clear. My name is XXXX XXXX. On XX/XX/XXXX, I was admitted to the ER and was treated and discharged the same day. I have insurance with XXXX, and I was later billed. I received a bill on or around XX/XX/XXXX from HCFS Healthcare Financial Services. The bill shows the following : ER Visit, Provider : XXXX XXXX XXXX, MD, debits {$500.00} on XX/XX/XXXX. The bill also shows " contractual adjustment '', a credit, of {$420.00}. This appears to be the medical insurance covering a portion of the bill ( XXXX is my medical provider ). The remaining amount due was listed as {$79.00}. I prompted paid the bill with my Health Savings Account ( HSA ) on or around XX/XX/XXXX, with a confirmation number of # XXXX. I assumed the debt was settled in full at this point. 
Approximately 2 years later, I am notified by my parents that they received a threatening letter stating that they ( HRRG, to be later named below ) would proceed with judgement ( lawsuit ) if I did not settle or dispute the debt within 30 days. This letter was dated XX/XX/XXXX, approximately 2 years after the debt was paid in full ( in my opinion ), without me being noticed of any subsequent action taken by either the insurance company, the hospital, the doctor, or any other party. 
I contacted the company Heathcare Revenue Recovery Group, LLC ( debt collector, also referenced above as HRRG ) on XX/XX/XXXX via my cell phone at approximately XXXX XXXX  EST. I spoke to an individual that refused to accept my documents related to the bill I had paid and continued to threaten legal action. I requested a supervisor and discussed my documents and understanding of the bill calmly with her. She advised me to send in the documentation and they would review and return my phone call if any further action was needed. I explained that I would pay the bill, if they provided me a document showing why I owed the amount, and why it was different from the bill I had previously paid. We ended the call with my assumption of the debt being settled unless I heard back from them. 
I was not contacted again by the company until XX/XX/XXXX at approximately XXXX XXXX EST, where an individual began the same type of conversation about the debt and he explained that they ( HRRG ) have sent all the necessary documents, but would not reveal where they had been sent. I have only received the original bill ( mentioned in the first paragraph ) and the final demand letter, which threatened judgement ( mentioned in the second paragraph ). The individual explained that they would send me a letter ( showing the justification for the debt ) within 5-10 business days, which is something that I requested almost 30 days ago ( on XX/XX/XXXX ). I do not feel that I owe this debt, I feel that this debt is not valid, furthermore I feel that 30-40 days is an unreasonable due diligence timeline for a debt collection company ( HRRG ) that falls under regulation by the Consumer Financial Protection Bureau ( CFPB ). 
If the company has already sent the document showing the adjustment, it should only take minutes to reprint and send to me. To which I would expect to see the date of the alleged adjustment and justification for the difference in the discrepancy of the two bills. To advise a consumer to wait an additional 10 days, leaves me further worried that they will stall in sending the validation letter in order to proceed with judgement. I currently have no debt with any individual or company and a well above average credit score. I feel that these threats made towards me are unwarranted and unjustified. I will gladly  settle any balance owed, if I can validate the money owed is in fact due to an insurance adjustment, that I will need to validate with the company, XXXX. 
Thank you for your time and investigation into this matter.