Understood, but they really should have confirmed if the price was acceptable before processing. But, OK, the agent may have misunderstood the question as a request. Not optimal, but that could happen. But having done so, refusing to tell the OP how much he had paid is just outright odd. I think because it's miles people might be giving the airline more leeway. Although legally there's a grey area on gift cards, miles and other forms of pre-payment, from a customer service point of view that would be a pretty shabby route to take. Seeing things from the customer's point of view (who had just paid cold, hard cash for miles for a specific purpose) there would be little difference between this and the agent charging the credit card to buy the appropriate miles for an upgrade, then refusing to say how much he had charged. I think we would all find that unacceptable, so we should also find it unacceptable that the agent took the assets of the OP without forming the sale contract, and then refused to report what that sale was.

I really think this is one to take further. They might restore some of the miles. My thinking is that OP could argue he WAS prepared to purchase the upgrade if the cost was similar to his usual upgrades. It wouldn't be unreasonable to argue that such a cost would represent value to OP and was the price the OP was prepared to pay.

Really, OP, I would write a polite letter. Don't expect something back, but at very least they might have a word with their staff on protocol (it sounds like this is more than a one-off).