At first glance, it seems plausible that Marsellus forgave Butch because Butch saved him from the pawnshop where they had been captured by Maynard and Zed. But now after reviewing the film several times I feel that this might not be an acceptable explanation.
Of course Butch did save Marsellus from that hellhole. But do not forget: Butch double-crossed Marsellus in the boxing match, and it was also Butch that got both of them into the pawnshop situation.
In the very beginning, Butch had already made a promise to Marsellus to lose the match and had taken Marsellus's money. So Marcellus had every good reason to seek vengeance upon Butch for his betrayal. And again, if not for chasing Butch, he would not possibly have become captured in the pawnshop. Indeed, it was Butch to blame for all of the trouble.
So why would Marsellus forgive Butch at all? I mean, if I were in his situation, I wouldn't have felt very comfortable to let Butch go free.