Should I trust the information given by the agent over the chat?

I upgraded one of my lines last year. and there was $400 discount off the purchase price at that time. The agent helped me to place the order over the chat. The agent said that he needs to put in a ticket for $400 discount, so I will receive the $400 discount. But I did not receive $400 discount. I contacted agent multiple times before, and was told that I will receive $400 discount. But I never did. I really do not have much time to contact the agent over and over again. It has been a while. I have the whole chat transcript to show the communication between me and the agent about $400 discount. How to get my $400 discount?