Town chairman David Sheepshanks and manager George Burley have both paid tribute to striker David Johnson as he leaves Portman Road.
The Town manager said: "David has been a tremendous acquisition for the club being leading goal scorer in the last three seasons and played a major part in the club gaining promotion to the Premiership. He has a been a first class professional since his arrival from Bury and we will miss him. Despite that I understand David's needs to play regular first team football."
David Sheepshanks went through the details of the transfer: "The deal has been a protracted one as we were not prepared to let him go for less than the £3 million guaranteed fee."
He went on: "We have agreed this transfer very reluctantly as David Johnson has been a most valuable contributor to Ipswich Town's year on year improvement, culminating in our promotion. There is no pressure to sell and we would not be selling him at all if it was not for the fact that he personally wants to move on now in order to play regular first team football."
Sheepshanks continued: "I have had personal meetings and discussions both with David Platt and Forest Chairman, Eric Barnes throughout the week as they endeavoured to meet our terms which they have finally done this afternoon. I am satisfied that all things considered this represents a good deal however nothing is finalised until he has both passed a medical and agreed personal terms."
The £500,000 'top-up' on the deal will be paid if Forest gain promotion to the Premiership. Town paid out on similar deals for Marcus Stewart and Jim Magilton in the summer.
Johnson has travelled to Nottingham to discuss terms and have his medical.