Hold your ground and do not pay.
This answer does not create an attorney client relationship between you and I. I am not your attorney unless we both sign a written contract that describes our relationship and terms of the representation. Any information provided to you here is not a substitute for the advice you need to pursue any legal matter. I advise you to retain the services of a local attorney before taking any legal action in this matter.
If the creditor has failed to provide the proper proof after you sent a letter, you may have a claim under the Fair Debt Collections Practices Act. You should reach out to a local law firm who handles these cases, especially due to the fact that there is a fee shift provision in the FDCPA which makes the creditor pay your attorney fees if you are successful. This is why many times they settle prior to filing a law suit.