It's what I would do in that situation. The DOT has put the onus on United to produce a correct receipt. Whether or not this is a reasonable request (it seems like it is; in practice, it's not so clear-cut), it's the regulation UA is required to obey. That's not to say that UA won't complain, although we've heard occasional good news from that kind of approach. I'd be prepared to escalate it to the DOT, though.

(UA's old receipt sort of met the requirement, by printing the correct price struck through with a disclaimer. The new receipt format unequivocally does not).