I received a letter from a collection agency claiming that they are the owner of an old debt and due to the time that passed they can't sue for the balance of $18K but would like to settle for $3.8K.
What should I do?
That's very strange. First they are admitting that they cannot come after you for the amount due but then are asking that you settle it? Please check your credit report to see if they have reported this delinquency. You may have protections under consumer and fair credit reporting laws, depending on the specific facts of your situation.
Talk to a lawyer. If the time for filing (statute of limitations) has passed, they can’t collect from you at all. Don’t do anything until you talk to a lawyer.