Nco ollection account?

Hi recently was contacted by nco long story short i asked them to validate the account the ok then i receved a call from someone stating the validated the account and it was mine. So i recenly sent them payment in full. Next week i received a letter saying that they couldnt be validated the account and that it would be closed and deleted from my credit and it was deleted that also they would be refunding my money. The account was not sold to nco it was only assigned to them. Have i restarted the sol by sending them payment even if they refund my money and can the company that assign them still sue me even if the sol ran out and they cant validate it ?