Fry confirmed Posh inserted a 20% sell-on clause in St Ledger's move to Deepdale in 2006 and the Republic of Ireland international has now joined Boro on loan.
He is expected to make that move into a permanent deal in the January transfer window and Fry is delighted.
He told the Peterborough Evening Telegraph: "It is a fantastic deal which means in the end we could receive well over £1million for the player.
"The chairman is happy, so I'm happy. We have already received a transfer fee, more money linked to appearances and another sum for Preston making the play-offs last season.
"I understand the permanent transfer to Middlesbrough has been agreed but they are in a similar position as to we were with Gaby Zakuani last season where the loan spell will end before the deal can go through."