Resetting IPAD after account info is lost

I have a company IPAD that I need to reset because the previous owner left the company and we are unable to retrieve the account information that he used to register the IPAD. After factory resetting it it won't let me register it with Apple because it says it has been previously registered with another account. How do I get the IPAD unregistered when I have none of the original info and since they were bought by my company I don't have a receipt to take to a tore.