Snape is a Halfblood?

AU. What if a misplaced Hogwarts letter revealed that Tobias Snape was in fact not a muggle, but a wizard? A man from the Ministry of Magic finds just that and goes back in time to fix a wrong from the past. But just how will that one incident effect Severus's life? Supports the theory that Credence is the grandfather of Snape. Features both pre-Marauders era and Marauders era.