Being charged for equipment that was returned

I was with Xfinity for almost 9 years started off with a rented modem till I had the money to buy one myself after I bought and setup my modem to make sure it worked went to the belmar store in Lakewood Co to return your modem wasn't given a receipt was told once it was scanned it would update your system but now that I've cancelled service I'm being charged for equipment I returned years ago