The Butler & Fiancee Did It

Someone stole Ambrose's cote de boeuf (an expensive steak) and later on they discover someone added poison to the steak with the intent to kill Ambrose. It turns out Kendra added the steak sauce to kill Ambrose so she can inherit his money, but didn't account for Anton stealing the steak to sell on the black market. They were going to use the money to start a new life together.

Example of:
Two Dun It