If you were making payments from a bank account you should be able to contact your banking institution and ask for a history of your account activity. Once you have obtained a history, make a note of the payments to the previous debt collector on your statements (highlight or underline the payments to the previous debt collector). You can then send a written dispute of your account to the new agency that is collecting the debt. You will want to send the proof of the payments along with a detailed explanation of the payment arrangement with the previous debt collector and what you believe the balance on the account should currently be. If the debt is being reported to a consumer reporting agency (CRA) you should also send a notice of dispute with the same accompanying information to the CRA.