Sprint Not honoring a promised offer Nationwide

I was promise by a sprint representative that I was going to receive my $15 dollar credit per phone ( 2 phones = $30)for the next 23 months. I have the evidence for the chat that this was promise and they have not been giving me what I was promised. I call and ask about this issue and I was transfer to a higher person (manager) and he told me that there is nothing that they could do and that this was an error from the sprint representative. I change my plan because the $30 credit for my two phones was a great deal. I am very disappointed with this situation and what is more frustrating is that the manager told me that there is nothing they could do. I ask if they could cancel my contract without any fee because was not my fault that they promise something and then they couldn't honored. The answer was that I have to pay around $1000 to cancel my contract. I have been with Sprint for almost three years and I have never paid late. I would like to be honored with what they promised at the beginning.