I don't really do phonetics so I don't see how the "u" in "but" can be described as "a". "a" is the sound in "bat". I'm aware that some phonetic symbols don't resemble the letter they represent so it could well be that that's throwing me off.
Anyway, the point is that we pronounce the vowel in Rus in the same way as we pronounce it in Belarus, right?
My point is that, in BrE, that's not the case. The "u" in "Russia" is not pronounced the same as the "u" in "Belarus".