Debt question - lump sum or validate debt

[dreammer]

What is the name of your state? Maine

My son received a letter from a law firm/collection agency in NY asking for payment of a debt to Fingerhut from about 1999. He does not remember what he bought or for how much since he has had a stroke he has limited memory. The debt according to them is $1100. I typed a letter he signed and I questioned the validity of the debt as he has proof of paying $800 to Fingerhut a couple of years ago. And I asked if they could tell me what he bought and when because I have no idea what he actually owed them.
A couple days later he received a letter from the law firm/coll agcy offering a lump sum payment in full for $500 (our letters probably crossed in the mail). A couple days after that he received a letter from the law firm/collec agcy telling him they were returning his file to the original creditor (fingerhut) because of his letter in which he questioned the validity of the debt.
Now the dilema is, do we wait for another letter from the original creditor and miss out on the lump sum offer? Because he probably owes them something. Or should we take advantage of the lump sum payment to get rid of the thing. Or would that be a bad idea because they say they are returning the file to the original creditor?

thanks for any help you can give.

dreammer