Monocle wrote on 12/18/16 at 18:00:51:
1. d4 d5 2.c4 dxc4 3.Nf3 Nf6 4.e3 e6 5.Bxc4 a6 6.O-O b5 7.Bd3 c5
This is a known line. After 8.a4 b4 9.e4, it's thought to be favorable for White. For example, Avrukh gives 9..Bb7 10.e5 Nd5 11.Nbd2 Nd7 12.Nc4 cxd4 13.Bg5 Qb8 14.Re1 h6 15.Bh4 Nc5 16.Bg3 as advantageous for White.
Generally speaking, the problem is that 8..b4 weakens the Queenside and, in particular, the c4 square. Unlike in the Meran, there is no Knight on c3 to kick, so Black doesn't get any compensation for playing ..b4 in the form of either a tempo or a misplaced White piece. White gets to play e4-e5 and Nb1-d2-c4, with chances to play for Nd6. If that happens, Black tends to be in trouble, since even if he can take the Nd6, White may end up with a strong pawn on d6.
All that said, there are obviously other moves that both sides can try, and it's a complicated enough line. It may well be that Black can equalize. See, for example, Grachev-Fier, Aeroflot 2011, as a possible Black approach. I personally don't like to play lines as Black that give White a clear and good plan to pursue, even if, with careful play, I can equalize. But YMMV and there is certainly room to explore this line further.