Dons existing transfer record fee was £250k paid for Kieran Agard in 2016 and Alex Gilbey in 2017.
Fry said: “MK chased Mo for a long time. They wanted him in January, but they couldn’t match our valuation.
“They kept trying again in the summer and they kept uppping their bid until reaching a number that made our chairman happy. It’s comfortably a transfer record fee for MK.
“Good luck to Mo and good luck to MK Dons He’s a good signing for the way they like to play. I’d expect them to have a very successful season.”
Posh paid a club record £1.3 million to bring Eisa to London Road from Championship club Bristol City so it’s likely they’ve still have suffered a rare financial hit on a striker.
Eisa started his Posh career in fantastic form with 14 League One goals before Christmas after forming a deadly partnership with Ivan Toney. He scored twice in a 4-0 win at MK in one of his first Posh appearances.
But a switch of formation to one up front and the subsequent form of Toney meant Eisa was largely left on the sidelines.
He scored just two League One goals last season, including a stunner on what turned out to be his final Posh appearance in the last game of last season at Doncaster, and finished with 21 goals in 69 games (45 starts) in all competitions for the club.