GET IT IN WRITING!
If you do not get it in writing, that the debt collector will settle for less than what is fully owed on the account, and you send them money, they may very well come after you for the difference.
Here's the scenario: you owe $1,000 on an old credit card. A few years pass and then you get sued. You call the debt collector and offer to settle. Let's say they offer $450 to settle on it. Before sending them any money, get it in writing. Even if they tell you that the offer is only good for today only, unless they're able to send you an e-mail or a fax, do not make a deal with them. It has to be in writing.
Or no deal.