A debt purchasing company contacts you claiming to have bought your old debt and demanding immediate payment
The debt purchaser implies that the sale of your debt creates new obligations — including possibly new terms — that you must accept. In general legal principles, the sale of a debt (cession) does not change the terms of the original agreement. The new creditor steps into the shoes of the old one — they inherit exactly the same rights and no more. They cannot add interest, change the amount, or modify terms simply because the debt has been sold.
After you respond, they may push back with these arguments. Members get the full rebuttal for each.