It is legal, because a creditor or their debt collector has no obligation to engage in any settlement discussions whatsoever, or to settle a debt under any terms other than payment in full when due. Since a creditor or its representative does not have to settle a debt, they can refuse to do so, at any time, for any reason. That said:1) If someone refuses to validate a debt, it may well be a scam--while you have to make your own decision, speaking personally, I would *never* settle an unvalidated debt unless I had independent reason to know it was good and valid (i.e. there is no question whatsoever in mind as to the validity, terms, amount, etc. of the debt) and I'd gladly "give up" the possibility of settlement to validate the debt, especially since--2) No matter what they say now, they, *of course* could agree to settle the debt later, at any time--there is nothing preventing them changing their mind.