I don't think UA will have any trouble getting all the 781 from Boeing on time when it want them. There is no need to make a deal with BR. Everyone is deferring delivery so there are plenty of production slots available for anyone that wants to take delivery on time or ahead of time.
BR will probably reduce capacity by returning the leased 77W when the lease expire. But they will probably continue to operate a sizable fleet of 77W because the cargo lift capacity. BR has big cargo operation at LAX for example and even with scheduled 10x weekly 77F they still need daily 3x 77W (pre-Covid schedule) to take all the pallet cargo. When PAX air traffic returns to semi-normal, I don't see BR reducing LAX to 789. They may do mixed ops with 77W and 789 to the same destination but key cargo stations like LAX will probably always get some 77W for the foreseeable future.