Phone Company Refuses to Give Promised Refund

I bought a phone from a popular phone company and sent the phone I owned at the time in as a trade-in to get credit off the phone. It ended up being that I spent $415 on the phone and received a trade in credit of $700 (the phone's full price was $1115). I then later returned this new phone as I did not like it. This company assured me that not only would I receive the $415 refund for what I spent on the phone (which they have indeed sent me), but that also I would receive an additional refund of $700, the value of the phone I traded in. I was told this 3 times by 3 different agents and have records of all 3 instances where this was communicated. Then, after the new phone was received and processed, the company told me that the $700 can only be redeemed as a promocode for their specific products, and not a refund back into my bank account as I was initially told. I called and argued this until I reached the highest-up executive possible, but no resolution was found. What do I do now?