"We are willing to make all necessary schedule adjustments and modifications to accommodate Butler's entry into the A-10 on July 1, 2012.
"The unexpected circumstances that precipitated Butler's request to move immediately were unfortunate, but we will welcome Butler into the A-10 as a valuable new member."
Bulldogs athletic director Barry Collier said it was in the school's best interest to reach an agreement with the Horizon League to leave now.
Horizon League spokesman Bill Benner declined comment.
Butler announced four weeks ago it was leaving the Horizon League for the A-10 but would not make the move until 2013. However, that changed when Horizon League presidents favored making the Bulldogs ineligible for the Horizon League championship - and automatic NCAA tournament berth - in 2012-13.
Butler and VCU, which left the Colonial Athletic Association, both join the A-10 this fall. VCU also decided to join the A-10 in 2012 instead of 2013 because it didn't want to experience a lame duck season when the Rams would not have been eligible for the CAA tournament.