Dell computers

Home I returned a lap top computer to Dell October 13 of this year having followed all their requirements to time and shipping.Now in December I was billed for a computer I did not own.Now I received a telephone call from the finance department of Dell telling me they have no notice of the return and will continue calling until I pay them.Then I was told I would be turned over to their COLLECTION AGENCEY if I did not pay. I have receipt of shipment paid in October 13 and also have the tracking order,yet I am subject to this abuse.I dont know how this will end but for me it is GOOD BY TO DELL