Samir Carruthers has joined League 1 leaders Sheffield United for an undisclosed fee.
The 23-year-old was out of contract at Stadium MK at the end of the season, and it is understood the fee was too good for Dons to turn down.
Carruthers moves to Bramall Lane on a three-and-a-half year deal, though it is also believed former boss Karl Robinson was interested in taking the former Aston Villa man to Charlton.
"When a player enters the final few months of his contract, there has to be a value on him," said Dons manger Robbie Neilson after the 0-0 draw with Chesterfield on Monday.
"On the financial side, that money can go back into the squad and gives us an opportunity to bring more players in or spend a bit more money on better quality. That was the key decision for me.
"Samir is a fantastic player, but the time of year it was and the time left on his contract, I had to make the decision and this is the right one."
Carruthers first signed for Dons on loan from Villa in 2013/14 before making his deal permanent that summer. He made 137 appearances for the club, scoring six goals.
Sheffield United manager Chris Wilder said: "We have tracked and monitored Samir for some time and he is a player who can grow with us, we are delighted that the Board have backed us with this level of investment.
"Samir is the right age, has a great pedigree from his time at Villa and MK and he is an exciting player. He is an attacking midfielder who can fit into a number of positions and we are looking forward to introducing him to our great group."