No, but there is pre-merger contract in which United committed to Trent XWB engines for its A350 order, plus "TotalCare" service (which itself is very lucrative). These contracts often have onerous cancellation/rescission provisions. I don't profess to know anything about the specifics of a proprietary, confidential business agreement between UA and RR, but at the time of the AIP (12/2009) United was barely four years removed from bankruptcy and in the midst of another restructuring, posting a billion-dollar annual loss, while publicly courting a merger with US or CO. At the same time, RR was booking lots of orders for the 787 and A350, and hadn't yet encountered the reliability woes associated with the Trent 1000 fan blade cracking. Comparatively, RR was in a stronger bargaining position. It would not surprise me if the deal included significant, rather ironclad penalties if UA elects to walk away.

The 787s are all GEnx-powered. Some discussion took place re: the Trent 1000 TEN for the 787-10s, but that went nowhere.