Product: Debt collection

Dear Sir/Madam, My name is XXXX XXXX XXXX -XXXX  of XXXX XXXX XXXX XXXX . 
We live in CA and on XX/XX/XXXX we received a collection letter from KSN Law firm ( dated XX/XX/XXXX ) which was hired by XXXX XXXX XXXX ( XXXX  ) claiming we have not paid HOA on our rental unit ( XXXX XXXX CT, XXXX XXXX ) from the beginning of XXXX. We proved XXXX XXXX XXXX wrong in every possible way. 
Our HOA is on autopay bill pay set up thru our bank which clearly states the Association name, account number, property address etc. .We provided our XXXX XXXX XXXX  proof that not only XXXX were regularly receiving our monthly HOA checks, but they were regularly endorsing and depositing it too! 
When provided with this undeniable proof, they blamed us for putting the wrong account number ( this number was provided by XXXX in the mailed letter to us XX/XX/XXXX ), otherwise it makes no sense how we will come up with this number. They then claimed that XXXX had no idea that we were based in CA. 
We provided proof negating this lie too, not only XXXX has a copy of our rental leases, insurance information etc. in the name of XXXX XXXX XXXX XXXX, which is based in CA, they also have our contact info, our emails on it. 
In fact they have been regularly emailing us about general association reminders etc., mailing us general association letters at our CA residence since XXXX. Many emails we delete as we go, but we still have few saved as proof. ( Please see proofs attached at the bottom ) Mr. XXXX XXXX XXXX from The KSN Law firm not only comes across as rude and aggressive, but he is not ready to acknowledge the facts and proofs when provided. 
Mr. XXXX XXXX had no answer to my logical questions that if XXXX are now claiming that my bank sent HOA checks with the wrong account number, then how come they quietly deposited this unaccounted money as free money in their other account. 
Also isn't it their basic duty to cross check all the facts on the HOA cheque 's before depositing? Why didn't they send us reminders at our CA residence where XXXX XXXX XXXX XXXX is based? Why didn't they email us? 

Also most ironic, KSN Law Firm sent us a collection letter legal notice in the name of XXXX XXXX XXXX XXXX at our CA address, both of these facts which XXXX claims it had no knowledge about!! 

But Mr XXXX is not ready to accept any legal facts and is only trying to put the blame on me this way or that and is just concerned about their legal fees which they have demanded I MUST PAY within the next 7 days! 
It's so unfair and unethical to me, I feel I'm being punished for paying my taxes, HOA on time like a responsible citizen and paying for XXXX XXXX XXXX  ( XXXX  ) gross negligence /mistakes/fraud? 
I perceive this as implied imminent threat, if i don't pay their unfair legal fees of {$630.00}, they will report it to credit bureau. Seems like extortion and unholy nexus at this point to me. 
This will negatively affect my new budding business in a very negative way for 7 years specially in this Pandemic when all small businesses are struggling to survive. 

Sincerely request you to investigate this please urgently before it gets reported to my credit score. 

Sincerely and Regards, XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX, XXXX CA XXXX XXXX XXXX please see attached - KSN collection letter mailed at our CA address in the name of XXXX XXXX XXXX XXXX Bank proof of XXXX checks endorsed and deposited by XXXX XXXX XXXX Proof of email which clearly states that we informed them about our mailing address ( this apart from the initial paperwork we filled out in XXXX welcome packet, wherein we provided copies of our rental leases, home insurance info etc. ) KSN LAW CONTACT XXXX XXXX XXXX XXXX XXXX XXXX, XXXX XXXX XXXX XXXX, Collection XXXX XXXX XXXX XXXX, IL XXXX T : XXXX F : XXXX XXXX : XXXX XXXX