> if I…pay UHG $18,000 and… UHG pays out $2,500…, only the remaining $15,500 should be counted as UHG revenue

To illustrate the problem with this, what would you calculate their revenue to be if you become severely ill and they pay out $100,000?

There is no such concept in accounting as negative gross revenue. And situations where net revenue goes negative are exceedingly rare and complex (you’d probably hear about it in the news and someone might end up in jail).