It would be helpful to have a lot more facts, but my first question would be whether or not you actually owe this money and for what. If you actually owe it ($6K or so) they are not required to accept less and are within their rights to ding your credit. Depending on what it was for and under what circumstances you might have certain defenses, but you sound as if you acknowledge the underlying obligation.
If you do find a way to pay less (their website for instance) they can still report the matte as "settled" which isn't a great thing to have on your credit report either.
If you decide to pay it on line, make sure you get a confirmation number. I'm assuming that would mean you are paying by credit card, so be sure to save the bill that such a charge comes on.Ask a similar question