Thanks after 3 calls to UA (first agent at the Refund dept was confused and I'm pretty sure intentionally hung up on me rather than trying to understand) + 60 minutes on hold, I was provided a travel certificate by United to cover the difference. Not ideal, but better than having to fight with an unreasonable bank and the seemingly confused United Refund and Customer Service departments. The Global Services desk understood my frustration and simply offered up the certificate as a compromise. I'll live with it.