This situation sucks but it won't honor the verbal agreement, there's not much that can be done besides trying to escalate.
As others alluded to, trade in the more expensive car and sell the other. If the total was $58k and we use hypothetical $33k (trade-in) and $25k (private sale) values, the $25k times the tax is the "extra" tax that you would be paying which should be much less than the ~3k.