I was playing a game against a FIDE master which started with the Polish defence, Spassky gambit: 1. d4 b5 2. e4 Bb7 3. Bxb5 f5
[White "Bad_Bishop"] [FEN ""] 1. d4 b5 2. e4 Bb7 3. Bxb5 f5
What are the main plans and variations?
I have the book "1.d4 Volume 2" by Grandmaster Boris Avrukh in front of me. Not the line you're looking at but Avrukh says 3.Bxb5 Bxe4 4.Nf3 Nf6 5.0-0 e6 6. c4 Be7 7.Nc3 Bb7 is perfectly playable for Black.
Avrukh's mainline vs the Polish defence is 1. d4 b5 2. e4 Bb7 3. Bd3 (solidifying the centre and aiming to prove the centre pawns are better than the wing pawns.) Nf6 (3...a6 4.Nf3 transposes to the St. George defense) 4. Nd2 e6 5. Ngf3 a6 6. 0-0 c5 7.dc Bxc5 8. e5 Nd5 9. Ne4 Be7 10. a4 b4 11.Nfd2! (heading for c4 and and freeing the Queen on the d1-h5 diagonal) ... and eventually += on move 18. The main plan for White is to play with pieces in the free space in the centre and possibly aim to attack on the Kingside or fill the holes left behind in Black's pawn structure. For more details buy the book.
Your best bet here is 4.Nh3 so your opponent doesn't get a tempo with 4. ...fxe4
If he takes with his pawn 4. ...fxe4 You can simply continue 5.Nc3 followed by 0-0 and you're clearly better.
If he takes with his bishop 4. ... Bxe4 you can with no fear proceed with 0-0 and your knight will always pose a threat such as Ng5 attacking his bishop and with pressure on f7 and even e6 if he moves his d pawn, or Nf4 where your knight gets a secure,strong position but in that case you should be careful about his g pawn advancing. All things considered white is better in this position.
I tried the line 1.d4 b5 2.e4 Bb7 3.Bxb5 f5 4.exf5 Bxg2 5.Qh5+ g6 6.fxg6 Bg7 7.gxh7+ Kf8 8.hxg8=Q+ Kxg8 9.Qg6 Bxh1 from the comments & gained dynamic equality.
The line from 1. d4 Volume 2 given by @magd gives a game more in my style & less in my opponent's, but only with a slight edge.