This is a common tactic used by collection agencies. They attempt to overcome the SOL by getting people to acknowledge a past debt by saying that they'll forgive most of the debt if you merely pay a portion of it. When confronted with such an "offer" you should simply tell them to stop calling you and hang up. If they continue to call, they would be violating the Federal Fair Debt Collections Act. Further, if they end up reporting the past debt to a credit reporting agency, and the debt truly is beyond the SOL, they would also be in violation of the Federal Fair Credit Reporting Act. Beyond this, your state may have additional laws protecting consumers against practices like this. You should consult with a debt collection attorney and stop having any communications with the collection agency.

