After Butch breaks free, he makes the honorable decision to go back and save his arch-enemy and his butthole. In exchange for saving his life, Marsellus forgives Butch for double-crossing him as long as he 1) leaves town forever, and 2) never tells anyone what just happened.
"Actually, this is so improbable that no one would believe you, so go right ahead."
John Travolta. Or, more specifically, Travolta's character's corpse in Butch's bathroom. Remember, right before the pawn shop sequence, Butch stops by his apartment and finds Marsellus' top henchman, Vincent Vega (Travolta), using his toilet without asking, and kills him right there.
"I smell Pop-Tarts!"
When you're watching the movie, it's easy to assume that Marcellus is forgiving Butch for this, too. But Marcellus didn't know about it -- Butch runs into him (literally) on his way back to the apartment. When Vincent's bullet-riddled body is found inside Butch's bathroom, it's going to be pretty obvious who was responsible -- especially since Marsellus already knows Butch was at his apartment, since they had their encounter right outside the building.
"Usually my other henchman gets the doughnuts, but he just quit to become a drifter."