Err... question is a month old... but a coincidence... Warren mentioned this just yesterday in FAQ Friday!
Send the mastering engineer the reference that the client has heard / approved, as well as the flat (unprocessed) mix. He said v occasionally a mastering engineer might just tweak the approved mix, if they like it, but giving them a view of the mix the client has approved means they're not working blind.
>>If I bypass the master bus, the mix sounds completely different.
If you're doing multiband and changing the mix balance, maybe send them the setting? You could create a project with only raw stems, and the same master channel (with settings intact), as well as screenshots?
OT - as I'm sure you're not in this mould - but Warren's advice was "always send people as much as you can, to help them be successful, don't hold things back". It's great advice - I've had some experience trying to drag elements out of the hands of engineers who are paranoid about their IP... so, after a while, I try not to work with those folks, not just because their behaviour is disingenuous, but because the end result (and the customer) always suffers...