What they sold the debt for is irrelevant. It doesn't change the amount you owe, just who you pay it to.
The process of selling debt ahead of its maturity is an old, well founded practice called 'factoring'. No one would ever buy the debt for its face value in a commercial situation. It's a gamble as to if/when the payment will come, and how much it will be. If you went bankrupt tomorrow, they'd get nothing.
You might be able to get an order for a copy of the document, but the point of it would be to confirm that you owe the debt collector the money.