For starters, Black is best off not taking the pawn. I assume you mean 4.Nc3, against which 4...d6 scores really well for Black rather than 4...Qxb2.
London System early c5-line
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Basically, to open the diagonal for the DSB and attack on the queenside with rook/knight/bishop after playing c4.
Many times (in 1.d4 games) an issue for White is where the DSB belongs. Usually, it isn't d2, and sometimes c1 is the place to be for a while.
Qb6 tries to prove this point.
Another ongoing thread (somewhat related)
https://www.chess.com/forum/view/chess-openings/what-to-play-against-the-old-benoni
Sorry for confusing post. I left out a some moves. In the image attached we've reached there by the following moves:
1.d4 Nf6 2. Bf4 c5 3. e3 Qb6 4. Nc3 Qxb2 5. Nb5 Nd5 6. a3 a6 7. Rb1 Qa2 8. Qc1 axb5 9. Ra1 Qxa1 10. Qxa1 e5. It's the 10... e5 move I don't understand. It gives white a pawn.
herr_shaibel wrote:
Sorry for confusing post. I left out a some moves. In the image attached we've reached there by the following moves:
1.d4 Nf6 2. Bf4 c5 3. e3 Qb6 4. Nc3 Qxb2 5. Nb5 Nd5 6. a3 a6 7. Rb1 Qa2 8. Qc1 axb5 9. Ra1 Qxa1 10. Qxa1 e5. It's the 10... e5 move I don't understand. It gives white a pawn.
I'm with pfren on not take the pawn after 4.Nc3, but with 4.Na3 and I'd fulfill the threat and look to draw by the simpler means. At higher levels, you might look to complicate things (looking for a win) and follow up with 6...e6 (after the line he gave with 4...cxd4)
Here is the whole episode "London System against an early c5": https://youtu.be/N9CczxstD2g I do understand that black should never go Qxb2. It's the 10... e5 that seems unnecessary.
#8
10...e5 is to open the diagonal of Bf8 and to prepare ...O-O
Black should take 4...Qxb2 the poisoned pawn on b2, otherwise 3...Qb6 has no point.
tygxc wrote:
#8
10...e5 is to open the diagonal of Bf8 and to prepare ...O-O
Black should take 4...Qxb2 the poisoned pawn on b2, otherwise 3...Qb6 has no point.
The other point was cxd4
Edit: video referenced (12:00 ish)
pfren, thanks, I understand now. Black need to create havoc as long as the king is exposed and white is undeveloped on the kingside. On the other hand black has some pieces hanging, attacked by the queen and the dark-squared bishop.
pfren wrote:
tygxc wrote:
#8
10...e5 is to open the diagonal of Bf8 and to prepare ...O-O
Black should take 4...Qxb2 the poisoned pawn on b2, otherwise 3...Qb6 has no point.
No, 10...e5! is definitely not played to prepare castling. Far from it.
An about taking the b2 pawn: Ever heard of a patzer named Aron Nimzowitz, who claimed that the threat is stronger than the execution?
4...Qxb2 is not a move Black should play, first because it forces Black to play several "only moves" in the process to stay alive, and second because it allows "games" like the following one:
Even faster than the famous Berlin draw. 
Wow, the white queen does everything there.
herr_shaibel wrote:
pfren, thanks, I understand now. Black need to create havoc as long as the king is exposed and white is undeveloped on the kingside. On the other hand black has some pieces hanging, attacked by the queen and the dark-squared bishop.
He's showing you how to fight for winning chances.
I (and Hanging Pawns) was showing how to draw with Black playing after the queen exchange (I thought). The video was for White (London System vs Benoni set up and playing e3). The game pfren posted (#18) is interesting (16.g4 I never considered) and a deep analysis would be nice (if only that game were annotated).
I also sort overlooked the safe square (d4) for White's queen. I focused on the route of 11...Nxf4 12.exf4 c4
I'm sure it's very difficult either way. But perhaps for both sides at lower levels (not just Black).
Thanks @pfren for sharing valuable and concrete insights/games
Yes, thanks for the heavy lift. That was awesome feedback. gg <3 
Thanks everyone for sharing. Good thread
Game was a draw, but the engine didn't prefer 10... e5. 
Use the online engine with extreme caution (don't). If you have no choice, use self analysis and unlimited depth (sometimes the real key). And remember there are positions engines just don't understand at all. The ICCF game pfren shared is a good example to compare by using multiple lines (3 might be the max).
This is a theorethical line starting with 1. d4 Nf6 2. Bf4 c5 3. e3 Qb6. Black is attacking the b2 pawn but has to give up the queen for a knight and a rook. After that black should play e5, and I really don't get why send out the pawn unprotected like this (from Hanging Pawns). Any thoughts?