Told $500 Credit for New Phone and Not Given Credit

Good afternoon.  I have tried multiple times to get what was told to me on a transaction done online with a Sales Agent.  I started this back at the end of February and mentioned twice in my chat that I had a broken phone and the agent said - "No Worries - I will take care of you"  I paid off the remaining amount due on the old phone to go through with the transaction as well as did not go to Verizon or T-Mobile that was running the "We will take back any phone in any condition" and gotten the phone for free.  I currently have Mobile, Internet, and TV through Comcast but I guess that doesn't matter!  Let me pay $400 or so a month and just be told that we are sorry but the agent was wrong and we will talk to that agent but not give you credit for what you were told.  I have called 3 times to speak to someone and the highest they send me is a Supervisor which is ridiculous.  I guess those that sit upstairs never have to hear what is going on downstairs - the water will never make it to my floor belief!  I am only asking for what was told by the agent, TWICE in the chat - $500 off the phone.  A $20.83 credit for the 24 months - I am not asking for the world!