I'm currently trying to get the name of the 3rd party vendor. Two different representatives have indicated they're not allowed to share that information, feedback which was forwarded on to the DoT.
Things happen in patterns. To date, I have never heard of an airline admitting a mistake in currency conversion that resulted in a consumer paying too much, prompting an automatic FX adjustment refund to the customer. This exposed the fact that errors are possible and the only reason they were noticed today is because the discrepancy was so large. How many times has it happened before and no one knew and paid too much? Easiest way would be to contact the 3rd party vendor and demand a flat file of their historical fx rates to compare against an industry standard (Bloomberg, IDC, etc.).
If United is using the third party vendor as a shield against honoring the fares, they should be willing to provide that information.