Equipment Return Receipt

I cancelled service effective yesterday and returned my modem to an Xfinity store yesterday, and they said they could not give me a receipt.  I called the Xfinity phone support and was told to go back to the store for a receipt.  I went back today and spoke with the manager and he said they could not give me a receipt as the account was closed.  This is not a good business practice in my opinion as I have no proof of return.  Please help.