Send it via Certified Mail, return Receipt Requested, and save the signed green card when you get it back.
Here is the DebtCC Debt Validation page with complete explanations, and sample letters. Tailor accordingly.
There are quite a few bad reports on this agency, that is why I hesitate to suggest you write to them directly, but through the CRA, because there is more oversight over the process. (The FTC randomly audits the CRA's Dispute files because of so many complaints of CRAs ignoring disputes or just passing it on to the alleged creditor who says "it's valid" and they say "Ok".)
But, you should be getting a dunning letter (describing the amount of the debt and it's source, and settlement offers). At any rate, the Certified letter may come in handy if they decide to get nasty about it.