Situation analysis

In this situation
snag.gy/Ssxly6.jpg

Why is it a mistake, or like it says an inaccuracy to play Nd2?
I did not want to take while non of my pieces was able to take again when he takes back... and if he takes next move, I wanted more then just my queen defending the pawn when I take back... I dont like my queen to be babysitting pawns.

with bxc4 you would have exchanged a wing pawn for a centre pawn, which is a good trade. If he takes back dxc4, you get control of the centre with e4.
After Nd2 he is not obliged to take cxb3, exchanging his c pawn for your a pawn.

Hmmm, did not think about it this way, thanks :)