With white threatening Ba7 winning black's queen, I think black has to play 1. .....Qb8 here to exchange the bishop for that pawn. Neither of the mate threats 1. ....Ba3 or 1. .....Ba1 are playable (1......cxd5 runs into 2.Ba7 I mentioned above)
2.Rd2! Qb8 (what else?)
3.Ka1 Qb7 (anything hold?)
And white's edge should be decisive. My only concern would be that I am missing a perpetual here, but I think white would find a way to avoid in this position fairly easily. More testing is 1......Ba3. At first, I didn't find white's best move- I thought he had to play 2.Rd2, but then it hit me....
2.Bd4!! Qb8 (what better?)
3.Re1! Kf8 (Kf7 4.Rxd7 wins)
5.Ka1! and white suddenly has the threat of Bf6 and a powerful mating attack...
5...........Qa8 (nothing better, I think)
It will do no good to vacate the queen off the 8th rank because of Bg7+ followed by mate. Continuing:
8.Bd8 wins pretty handily.
Finally, in the alternatives, 1......cxd5 is playable, I think, but the ending looks lost to my eye:
2.Ba7! Bd4 (best I hope)
5.Rc1 and the passed a-pawn should be decisive here with the rook edge.
So, as I wrote above, this seems to leave the simpler...
Not necessarily an easy move to find, I think- I missed it for several minutes trying to see if I could make 2.Bg5+ work, but still a bit obvious given the pawn at a2...
I tried different variations here trying to maintain the bishop, but all of them allowed white to keep the pawn on b7- for example 2....f4 3.Bc5+! d6 4.Rb3! and that just looks losing to me- the queen is tied down the rest of the way. Continuing:
And this is unclear to me, but I would rather be white here for sure. It might be better to play 4.Rxb2, but I can't tell the difference without a lot more work, and I am not sure I could prove one way or the other easily.