Cash upgrades at checkin/gate are possible on award tickets, its basically a reseating in the departure control system and the cash collected is documented in form of an EMD. Agents looking at the reservation will not see this process, so United agents should have known not to tell you what they did. Without screenshots you probably have a weak case asking for DOT assistance, CC dispute or the like.

Can't find the sccreenshot on how the offer looks like on LH.com, but the LX version (of the same system) is here: Upgrade during online check-in - new feature?

Note that the screen says 'from xxx EUR per person per flight'. So buyer beware.