Black has a few choices here:

(a) 6...c5 is uncommon but I think Black is fine after 7.d5 exd5 8.cxd5 Bxf1.

(b) 6...h5!? suits enterprising players, forcing 7.h4 and intending pressure against the White kingside after 7...Bb7 8.Bd2 Bd6, but obviously four bishop moves by move 8 come at a cost in development.

(c) I don't like 6...O-O much but it is a popular choice, often continuing 7.e4 d5 8.cxd5 Bxf1 9.Kxf1.

(d) 6...Bxc3+ 7.bxc3 d5 is the main line. White has two important choices here. 8.Qf3 is less common, with only a minimal White edge in my opinion after 8...O-O 9.cxd5 exd5 10.Bxa6 Nxa6. The more common choice is 8.Ba3, where White is willing to gambit the c4 pawn to prevent kingside castling. White gets excellent central compensation after 8...dxc4 9.e4 Qd7 10.Be2 Nc6 11.O-O O-O-O 12.Qc2 h5 13.Rfd1 (6 report games), so Black usually plays 8...Bxc4 9.Bxc4 dxc4. Here White can regain the pawn (and have a small queenless middlegame edge) with 10.Qa4+ Qd7 11.Qxc4 Qc6 12.Qxc6+ Nxc6 13.c4, or play 10.e4 Qd7 11.O-O with perhaps not quite enough compensation, since the light-squared bishops have left the board.

1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 e6 5.g3 dxc4 6.Bg2 b5 7.Ne5 Nd5 8.a4 is a Catalan position often reached from Semi-Slav move orders. Here Black played the new 8...b4!? with 9.Ne4 Be7 but perhaps 9...c3!? is the logical continuation, aiming to lose back the extra c-pawn in the most disruptive way possible. Previous moves here were 8...Bb4 (with several wins to White) and 8...f6 (Gelfand-Shirov, Paris 1992).