user3405: Let's say the position looks like this:
[Title "Black to move"]
[FEN ""]

 1.d4 Nf6 2. c4 c5 dxc5

I usually do this:
[Title "Black to move"]
[FEN ""]

1.d4 Nf6 2. c4 c5 3. dxc5 e6 4. b4 a5

I have played this position only once, and won, but still this might repeat in a open tournament I might go soon, so I need help to figure this out, because in the videos I have seen for the Benoni, I can't really find this variation anywhere.

user19861: The line you play is fine. 
After 5.b5, Bxc5 the position favors black.
bxa5 leaves white with a crap pawn structure. 

a3 just loses a pawn for white 

White has nothing after 4...a5

user2042: 
In the modern benoni, what should I do if this happens

Your continuation is the strongest in my opinion. I would advise you not to change anything.

because in the videos I have seen for the Benoni, I can't really find this variation anywhere.

Of course, because it is weak. The best White can do after your move, in my opinion is this:
[Title "BenOni"]
[fen ""]

1.d4 Nf6 2.c4 c5 3.dxc5 e6 4.b4 a5! 5.b5 Bxc5 

Here, I would aim for Bb2 + e3 + Nf3 + Be2 setup as White. Not ideal, but surely playable and the nearest towards equality I can get after that horrible b4...
If White tries to be "smart" about keeping the extra pawn, here is how I would play as Black:
[Title "BenOni"]
[fen ""]

1.d4 Nf6 2.c4 c5 3.dxc5 e6 4.b4 a5! $40 5.Bd2 ( 5.Qb3 axb4! 6.Qxb4 Na6! ) ( 5.Qa4 Na6! 6.a3? ( 6.Bd2? axb4 7.Bxb4 Nxb4! 8.Qxa8 Nc2+ 9.Kd2 Nxa1 $17 ) 6...axb4! $17 7.axb4?? Nc7! $19 ) 5...axb4! 6.Bxb4 Na6! $32 $40 7.Ba3 Nxc5 8.Bb2!? ( 8.Nf3? Rxa3! 9.Nxa3 Qa5+ 10.Qd2 Qxa3 $17)

Your only problem after b5 will be that you must play ...d5 which turns the game into positional battle. There will be no wild tactical positions anymore ( but still you will stand better ) so you will have to adapt psychologically to this fact ( tactical players hate "dry" positions and positional ones hate "wild" positions ). Try not to fall for this psychological trick. That is my last advice and a warning.
Good luck on the tournament!

3405: <p>Let's say the position looks like this:</p>

<pre><code>[Title "Black to move"]
[FEN ""]

 1.d4 Nf6 2. c4 c5 dxc5
</code></pre>

<p>I usually do this:</p>

<pre><code>[Title "Black to move"]
[FEN ""]

1.d4 Nf6 2. c4 c5 3. dxc5 e6 4. b4 a5
</code></pre>

<p>I have played this position only once, and won, but still this might repeat in a open tournament I might go soon, so I need help to figure this out, because in the videos I have seen for the Benoni, I can't really find this variation anywhere.</p>
 19861: <p>The line you play is fine. </p>

<p>After 5.b5, Bxc5 the position favors black.</p>

<p>bxa5 leaves white with a crap pawn structure. </p>

<ol start="5">
<li>a3 just loses a pawn for white </li>
</ol>

<p>White has nothing after 4...a5</p>
 2042: <blockquote>
  <p>In the modern benoni, what should I do if this happens</p>
</blockquote>

<p>Your continuation is the strongest in my opinion. I would advise you not to change anything.</p>

<blockquote>
  <p>because in the videos I have seen for the Benoni, I can't really find this variation anywhere.</p>
</blockquote>

<p>Of course, because it is weak. The best White can do after your move, <strong>in my opinion</strong> is this:</p>

<pre><code>[Title "BenOni"]
[fen ""]

1.d4 Nf6 2.c4 c5 3.dxc5 e6 4.b4 a5! 5.b5 Bxc5 
</code></pre>

<p>Here, I would aim for <code>Bb2 + e3 + Nf3 + Be2</code> setup as White. Not ideal, but surely playable and the nearest towards equality I can get after that horrible <code>b4</code>...</p>

<p>If White tries to be "smart" about keeping the extra pawn, here is how I would play as Black:</p>

<pre><code>[Title "BenOni"]
[fen ""]

1.d4 Nf6 2.c4 c5 3.dxc5 e6 4.b4 a5! $40 5.Bd2 ( 5.Qb3 axb4! 6.Qxb4 Na6! ) ( 5.Qa4 Na6! 6.a3? ( 6.Bd2? axb4 7.Bxb4 Nxb4! 8.Qxa8 Nc2+ 9.Kd2 Nxa1 $17 ) 6...axb4! $17 7.axb4?? Nc7! $19 ) 5...axb4! 6.Bxb4 Na6! $32 $40 7.Ba3 Nxc5 8.Bb2!? ( 8.Nf3? Rxa3! 9.Nxa3 Qa5+ 10.Qd2 Qxa3 $17)
</code></pre>

<p>Your only problem after <code>b5</code> will be that you must play <code>...d5</code> which turns the game into positional battle. There will be no wild tactical positions anymore ( but still you will stand better ) so you will have to adapt <strong>psychologically</strong> to this fact ( tactical players hate "dry" positions and positional ones hate "wild" positions ). Try not to fall for this psychological trick. That is my last advice and a warning.</p>

<p>Good luck on the tournament!</p>
