The earliest possible knight fork I hope it's OK with a question on the fringes of existing opening theory.

If white wants to combine pawn moves f4 and b4 early in the opening, he has to be aware of  the possibility of black snatching a pawn. One example is Larsen-Raizman from the 1958 Olympiad 1.f4 Nf6 2.Nf3 g6 3.b4, where black could have gone 3...Nd5!? with complicated play.

Even earlier possible pawn losses are 1.f4 d5 2.b4 Qd6 and 1.f4 Nf6 2.b4 Nd5. I want to focus on this last variation.

    [FEN ""]
1.f4 Nf6 2.b4 Nd5


Before I thought this was good for black, but now I'm not so sure anymore.
3.e4 Nxf4 4.d4 looks like great compensation, although ironically I would prefer the b4 pawn back on b2. Black is better advised to play 3...Nxb4, for instance 4.d4 d5. A possible improvement for white is 3.d4 and after 3...Nxb4 start chasing the knight around.

Questions:

Is 2...Nd5 advisable for black?

How should white react?
This question led me to spend two hours of my life looking at a line that I most likely never will get to play.  I went into it being quite confident that Nd5 was a bad move simply because White would get too much development for the cost of a simple pawn.  Analysing back and forth with Houdini has nevertheless convinced me that Nd5 is perfectly playable.

The similarities to the Sokolsky Opening

To get a feel for the b4 pawn it helps to understand a variation from the Sokolsky Opening.  It goes like this:

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1. b4 e6  2. Bb2 Nf6  3.b5


Here the pawn on b5 is slightly annoying.  It cannot be easily removed as a6 can be answered by a4 (it does not matter if the rooks are swapped off as White intends to castle short anyway) and c6 can be answered by e3 when Black must take away from the centre to remove the pawn.

In the game, however, Black can simply play a6 in the similar position as White can't answer with a4:

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1.f4 Nf6  2. b4 e6  3.b5 a6


So after 2...e6 White is only left with 3 a3, which, although reasonable, is another pawn move on the wing.  Black will be very solid after playing simple development like d5, Bd6 and 0-0.  This is slightly better than one of the main lines of the Sokolsky Opening, which is not that good to start with (but where White often ends up playing f4 later).

Houdini 3 on 2...Nd5

The currently strongest engine on the planet is Houdini 3.  When you let it analyse the position before 2...Nd5 to a depth of 25 it likes 2...d6 (claiming a small advantage for Black) the best (with 2...d5 as a strong second).  2...Nd5 does not come up as a top five alternative.  Houdini is initially not impressed by grabbing the pawn.

White plays 3 e4

Nevertheless, 2...Nd5 is still an interesting alternative and certainly gives more entertaining variations to look at.  First let us take a look at what happens if White plays 3 e4 after 2...Nd5 and then Black takes the pawn at b4:

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1.f4 Nf6  2.b4 Nd5  3.e4 Nxb4  4.c3 N4c6  5.d4 d5  6.Nd2 dxe4  7.Nxe4


To explain this variation it is important to know that Black's hope and main idea against a setup with f4-e4-d4 is to play d5 and develop Bc8 to f5or g4.  In the variation above it turns out Black can't play either.  Bg4 simply hangs the bishop and Bf5 runs into Ng3 chasing it back.  Here I (and Houdini) believe White has compensation for the pawn (although perhaps nothing more).

So, let us look at the alternative.  Initially I was sure that 3...Nxf4 must be horrible for Black, but it turns out it is good after 4. d4 e5!:

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1.f4 Nf6 2.b4 Nd5 3.e4 Nxf4 4.d4 e5 5.dxe5 Ne6


As seen above, if White makes the natural capture on e5, Black just retreats the knight.  Material is now even, but Black has two threats; The killing Qh4+ and just grabbing the b4 pawn.  So Black will end up a pawn up with a much better pawn structure and no imminent attack from White.

Instead White must play:

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1.f4 Nf6 2.b4 Nd5 3.e4 Nxf4 4.d4 e5 5.Nf3 Ne6 6.d5 Nd4 7.c3 (7.Ng5) Nxf3+ 8.Qxf3 a5


In the above variation please note that 6 Nxe5 runs into 6...Qxh4+.  The same idea works against 7 Nxd4.  And despite all the knight moves Black is left clearly better in this variation too.  If you want to play this variation with White you need to try the chaotic 7.Ng5!?, hoping to trap the knight at d4.  Have fun analysing that.

White plays 3 d4

I believe 3 d4 is a strong alternative for White.  If Black takes the pawn on b4 it transposes to a variation we have already looked at:

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1.f4 Nf6 2.b4 Nd5 3.d4 Nxb4 4.c3 N4c6 5.e4 d5 6.Nd2 dxe4 7.Nxe4


So, as I mentioned in a comment to the question, perhaps Black can try the amazing 3...Nf6 with the simple plan of d5 and Bf5.  In this variation White will have more space, but also more weaknesses (and, no, Black is not behind in development):

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1.f4 Nf6 2.b4 Nd5 3.d4 Nf6


White plays 3 Bb2

Seeing that White will lose one pawn anyway, why not play a development move like  and see which pawn Black decides to take?  It seems that Black can just grab the f4 pawn and get away with it.  Let us see what happens if White tries to push Black off the board:

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1.f4 Nf6 2.b4 Nd5 3.Bb2 Nxf4 4.e3 Ng6 5.h4 h5 6.Be2 d5 7.Bxh5 Qd6


White won the h5 pawn, but now the threat of Qg3+ is just killing White.  Instead, if White tried some quiet development moves:

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1.f4 Nf6 2.b4 Nd5 3.Bb2 Nxf4 4.e3 Ng6 5.Nf3 e5 6.Nxe5 Nxe5 7.Bxe5 Nc6


Black gets a small advantage by the precise 5...e5 which trades the not so optimally placed Ng6 (Black is still a pawn up).  Note that the alternative 3 Nf3 could easily transpose as it does not seem that White can improve much

Conclusion

The fork 2...Nd5 works out well for Black, despite Houdini's initial reluctance towards it.  It seems White should play 3 d4 to equalise.  I would certainly try 2...Nd5 should 1 f4 Nf6  2 b4 be played against me.