Soundness of the Tarrasch

it looks to me that the tarrasch defence of the queens gambit wins a pawn for white. can someone tell me what im missing?

After 5...Nf6, White usually gives the pawn back.

5.Nf6 and white is unwise to try to hold the pawn on c5. 6.b4? a5 6.Na4?? Qa4+ 6.Be3? and I guess black can play something like Qc7 regaining the pawn or just develop Nc6, Bf5 and clearly get a better position a pawn down because white has to untangle the kingside which takes several moves. Basically the pawn on c5 is not worth keeping. 

The move to play is 5...d4 e.g. 6.Na4 Bxc5 7. Nxc5 Qa5+

In fact, 5. dxc5 isn't the best, according to theory.

thanks guys. that really helped.

5.dxc5 is still viable though, and probably gives white some kind of edge since black has to give up his bishop pair to regain the pawn. That said, lines with a fianchetto followed by pressurising d5 are more challenging.

Actually, whites most common response for white is to go into the Rubinstein variation after 1.d4 d5 2.c4 e6 3.Nc3 c5 4.cxd5 exd5 5.Nf3 Nc6 6.g3 Nf6 7.Bg2 Be7 8.O-O O-O 9.Bg5 c4 10.Ne5 Be6 as recommended in The Tarrasch Defense by Jacob Aagard (it's a great book). However white is the one who gets into an inferior position after he takes on d4 because after 1.d4 d5 2.c4 e6 3.Nc3 c5 4.cxd5 exd5 5.dxc5 d4 (Nf6 is inferior for black, I have played hundreds of games with the tarrasch as black and have to say that this move is rediculous; d4 is a must in these kinds of positions.) 6. Na4 b5! Black gambits a pawn for initiative and the computer says it's better for black. Also, if you take on c5 with the bishop, you give up the bishop pair for no compensation and leave your queen in the center in the opening. 7.axb6 axb6 8.b3 b5 9. Nb2 (if 9.Qd3? Sets a trap for Qe4+, however it loses the castling rights after Bb4+ 10.Kd1 Nc6 and white is in a lot of trouble.) Nf6 (Bb4+ is fine also) 10.e3 Nc6 11. Nf3 Bb4+ 12. Bd2 Bxd2+ 13.Qxd2 Ne4 14. Qc2 Qa5+ 15. Nd2 Nc3 16.Bd3 Nb4 17.Qc1 O-O -+ and so on... Whites King is stuck in the center waiting for its impending predicament.