I would reach this via a QGA move order because I'm not satisfied with Black's other choices after 3.Nf3.  However to stick with the Slav move order:

1.d4 d5 2.c4 c6 3.Nf3 dxc4 4.a4 (second most popular) and here 4...Bf5 has almost never been played.  If White tries to play in similar fashion to the "Krause Attack", (which can be reached by transposition after 5.Nc3 Nf6 6.Ne5) with an immediate 5.Ne5, then 5...Nd7! 6.Nxc4 e5! 7.dex5 Nxe5 8.Qxd8+ Rxd8 9.Nxe5 Bxb1 getting the piece back.

After 5.e3 Black can likewise play 5...Nd7!? it's not nearly as good but does provide a system.  Or I can simply fall back on the "Czech" line of the Slav with 5...Nf6.   

With this move order I sidestep other Slav variations like the exchange, and lines in the QGA that I don't enjoy after 3.Nf3 Nf6.  But I get the benefit of possibly facing 3rd move alternatives in the QGA that aren't nearly as good as 3.Nf3