If this was in response to a letter you sent to the CA, I'd try the OC.
Do you know the SOL on this? If you're out from under the SOL, you can use that to your advantage.
Thank you for your letter of June X, 2007. blah, blah, blah. Since this debt is no longer within the SOL, then it seems a reasonable concession on your part to agree to delete this TL from all CRAs to which it is reported or ever has been reported. My financial situation is such that, without a deletion, there is no advantage to paying this debt. Simply marking it as paid will do nothing to help my credit score.
Don't offer any more money, yet. Save copies of everything you send them, save everything they send you, and save all CMRRR documentation. Occassionally, CAs will get stupid and fire off letters making threats--in clear violation of the FDCPA. Read a couple over on CB that were just priceless, but made me jealous I didn't have that idiot CA after me.