Spike's sent back to the beginning of season 4 and that tells Giles and Buffy he's from future.

I don't remember much about this fiction, only that Spike's sent back in time at the beginning of season4 and that tells everything to Buffy and Giles. Giles doesnt belive him though. Then everything Spike said would have happened starts to happen and Giles believe him. I dont remember if Spike has a soul in this one or not. I'm sorry if it's really vague but I cant remember anything else.

There are probably several that fit your description, but is it this one?

What You Wished For by pfeifferpack

Pretty much AU all the way, for obvious reasons. Starts in S4 at the beginning of "Goodbye, Iowa" then takes a quick turn to S1. Spike learns not only to be careful what you wish for, but also that when you swim in the Nile too long, you could drown.

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Guess it's found then since the OP hasn't replied