The amount of interest that a creditor can charge depends on what the contract says, subject to any limitation put on the amount of interest by state law.
When a debt is sold, what is sold is all the rights that the original lender might have against you.
Hope this perspective helps!
Unfortunately, if the Contract imposes interest, then you are liable for the interest. Same with the late fees. When the originating creditor sells the note, the purchaser obtains all the rights the original creditor had.
You need to talk to a TX attorney about what rights you have to defend; and you may want to talk to a bankruptcy attorney depending on how much actual debt you have.
Feel free to contact my office anytime on Monday, or email me. In my opinion, it is possible that this agency overshadowed your right to dispute the validity of the total charges of the debt. Yes you owe the debt, but who knows the the fees and interest that they wish to charge are valid? They could owe you max $1000.00 for violating the Fair Debt Collection Practices Act, and we know attorneys who might represent you at no cost out of your pocket.
Please do not take my answer to be legal advice that would establish any attorney-client relationship. Please take it as a general response from my own experience in response to your question. I hope you find it helpful.