$50 credit for returning inactive equipment

On 9/16/2020 I received an email offering a $50 credit if I return my inactive equipment by 9/28/2020. I returned the equipment and received a confirmation that my equipment was received on 9/23/2020. But I never received the $50 credit...the issue was raised on two different phonecalls to xfinity customer service but neither agent was willing to even look into the matter. How can I get this issue resolved?