Once a debt is settled

Paid an agreed upon amt to a creditor last wk. Did not get it in writing. This collector assured me that no other collection activity would go forward on the acct. A wk later, I recieve a letter in the mail for the entire amt! I contacted these people and spoke to a man at first, and before I could fully engage him in conversation, he hung up on me! called again and spoke to a woman. She stated that bill was not paid and that I owed the remaining balance and then she goes, Oh, wait...this acct was settled, you can disregard the letter. I cautioned her that if this "mixup" happened again, that I would speak to my attorney. She became persnipitty and stated well, miss so and so, we're trying to help you resolve the issue, you don't have to use threats! Threats...what threats? I was'nt yelling, or screaming or irrational. I just told her that I would consult my attorney if something like this happens again. I called back after this and got her assurances that they would not persue the remaining balance, and that I would recieve a reciept from them verifying payment. By the way, in the letters that were sent, there was nothing in either one of them about allotting 30 days for debt validation/dispute. I am still going to contact my attorney about the matter, because something fishy about this co. CPS (Marauder) or whatever they are calling themselves. Nasty bunch of people.