Here is the text from the FDCPA related to validation of debt.
In summary, the collection agency is not required to give "details" in response to a dispute/validation request. The primary federal appellate court cases have set a fairly low standard. Basically, so long as the verification includes the name of the original creditor, the amount claimed owed, and a statement that they verified the debt (along with what is mentioned below), that is generally enough. Many websites state that a debt collector is required to give you a statement of account etc., not the case. Some state versions of the FDCPA may require such, but the standard is pretty low on the FDCPA.
§ 809. Validation of debts [15 USC 1692g]
(a) Within five days after the initial communication with a consumer in connection with the collection of any debt, a debt collector shall, unless the following information is contained in the initial communication or the consumer has paid the debt, send the consumer a written notice containing --
(1) the amount of the debt;
(2) the name of the creditor to whom the debt is owed;
(3) a statement that unless the consumer, within thirty days after receipt of the notice, disputes the validity of the debt, or any portion thereof, the debt will be assumed to be valid by the debt collector;
(4) a statement that if the consumer notifies the debt collector in writing within the thirty-day period that the debt, or any portion thereof, is disputed, the debt collector will obtain verification of the debt or a copy of a judgment against the consumer and a copy of such verification or judgment will be mailed to the consumer by the debt collector; and
(5) a statement that, upon the consumer's written request within the thirty-day period, the debt collector will provide the consumer with the name and address of the original creditor, if different from the current creditor.
(b) If the consumer notifies the debt collector in writing within the thirty-day period described in subsection (a) that the debt, or any portion thereof, is disputed, or that the consumer requests the name and address of the original creditor, the debt collector shall cease collection of the debt, or any disputed portion thereof, until the debt collector obtains verification of the debt or any copy of a judgment, or the name and address of the original creditor, and a copy of such verification or judgment, or name and address of the original creditor, is mailed to the consumer by the debt collector.
(c) The failure of a consumer to dispute the validity of a debt under this section may not be construed by any court as an admission of liability by the consumer.
It sounds as if you have a valid contract for payment of the debt. If so and you made the payments, the creditor cannot unilaterally decide they no longer want to honor the contract - unless they are willing to breach the contract. I'd dispute the debt, tell them of the contract and provide proof of payment on the terms of that contract and ask (1) the matter be returned to the creditor and (2) no adverse credit action be taken. If they make an erroneous report on your credit report, and you were current, etc, you have a claim for breach of contract and a right to ask for the credit report to be corrected.
You may have a valid claim against the collection agency for violation of the Fair Debt Collection Practices Act, but you should have an attorney draft a letter to "set up" the collection agency, giving them an opportunity to correct the situation before you file a lawsuit against them.