Butler walked out on the National League North side after just one appearance to join Posh as a coach with special responsibilities for set pieces.
Boston refused to release him from his contract, but Butler continued to work at Posh and has been seen in the technical area at matches this season.
Today (September 3) Boston released a terse statement which claimed a breach of contract by Butler.
The statement read: “Boston United can confirm that key summer signing Andy Butler has left the club and joined the coaching staff at Peterborough United.
“We are extremely disappointed by this, as Andy is a contracted player to Boston United.
“This is a very unfortunate and unwelcome issue and and the club are trying to resolve the breach of contract situation with the player and Peterborough United.”
Posh are yet to comment.