Re: Jtixs QGD unfamiliar side line
The refutation to the ' American ' defence give in the books is 1.d4 Nf6 2.c4 d5?! 3.cxd5 gaining the centre Nxd5 4.Nf3! when at best, black gets a bad Grunfeld position. Anyway, that's what they say. Frank Marshall played it, which is likely where the name comes from.
Having read the above, you should not see it ever again
This is a common type of tactic. In the opening we need first of all to develop our pieces and get castled. Call this your INTENT, and you should never give up on it easily. The above tactic needs certain things for it to work. The knight move opens the a4-e8 diagonal, which suggests a double attack Qa4+. From this we deduce the knight MUST be defended when it arrives on b4. Black will due to his own development problem need to move his e-pawn allowing his bishop to move and so he can castle. Note only after the e-pawn move does the bishop control the b4-square, thus making Nb4 a viable tactic. Otherwise black would just lose time and have to block the queen check by moving his knight back to where it started.
From the above, I suggest ignoring the ' threat ' and continue developing with Bf4. You can answer any tricky Nb4 moves in it's tracks by just answering with Rc1 -another simple developing move.
If this was instant, he had likely pre-moved. ( set it to an answer to any move you make, so as to save clock time )
I did not like your Qb3. The threat of taking on b7 looks more risky for white than black. I do have a suspicion you transposed into the Baltic defence at some point. Of which I know zilch.