The Right to Validate Your Debt
Under the FDCPA, you are allowed to validate this debt, and the creditor (in this case, the collection agency) must show you proof that you owe the debt to the collection agency (not to the original creditor.)
The specific section of the FDCPA:
FDCPA Section 809. Validation of debts [15 USC 1692g]
(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.
Plus, they must show proof positive that you owe them this debt. It’s not enough to send you a computer-generated printout of the debt. There is an opinion letter from the FTC to back this up: