Should the clubs not be able to come to an agreement, the amount will be determined by a tribunal.
The 21-year-old left-back had been on loan at Elland road since January, and despite fitness problems giving him a wobbly start to his Leeds career, he soon became a hit with the fans after his winning free-kick at Doncaster.
Sheehan has signed a 3-year-deal at Leeds and will challenge Ben Parker for the left-back position next season.
McAllister told United's official website: "When we first brought him in he struggled a little bit, but he was the first to admit that.
"The jump from reserve team to first team may have been a bit too much, but he worked very hard, came back and did ever so well.
"One of the things that struck me was how hard he was prepared to work. He was determined to win a contract here and I was impressed by his attitude."