After you pay a creditor an agreed amount on a debt to be paid in full, can they ask for more money?

If you received a settlement offer, accepted its terms, and complied with its terms, then that offer was a binding contract between you and the debt collector and they would be bound to its terms. It is sometimes possible to overturn a contract if of either of the following existed: 1) mutual mistake--both parties thought it meant something other than what it did; or 2) unilateral mistake plus fraud--that is, you basically tricked them into agreeing to the wrong thing.

From what you write, it would appear than neither of these conditions existed; therefore, you'd have a good chance of holding them to this agreement. On the other hand, if they sue you, they *might* win--lawsuits are never certain--and defending yourself from a suit has its own costs. You may wish to consider whether it would be worthwhile taking the offer (make sure its firm in writing!) to avoid possibly being sued.