I recently played this takeback game with a 2300 player. I tried to gambit a pawn in the opening but it went badly.

You say you have to play c5, but why immediately?

b6, Na6, Nc6, dxc4, a5 and maybe even c6 are all useful candidates.

And if you play c5, why not follow up naturally with d4? The pawn can be defended with Nc6 and e5 and you get your pieces out very fast while threatening to open up attacking files with b6.

White's O-O-O is very dangerous as his g4-g5 attack is not necessarily faster than the black counter.

Probably, we don't need to take c5 pawn immediately in this line.

10. ...Bf6Xc3  

11. Qxc3 Qxa2 

I think the endgame after 29...Rxb4 might be hold. White is clearly better, but he has not won yet...

After 29... Rxb4 30 Ne7+ the outlook is pretty gloomy for Black.

Indeed... Well, never mind then 

Thanks for taking a look. The problem is that in this position if Black plays c5 the d-pawn is going to drop so I figured it was better to not wait and try to gambit the pawn for some development.

And I guess not taking the c-pawn so early was a better idea.

21...Bg4 was probably a bad idea but I really couldn't see anything else. The king can never move because the knight will start attacking pawns and the bishop is protecting my d-pawn.