Forrester signed for the Sky Bet Championship club during the summer when he was due to be out of contract. The Bees had offered the 22-year-old a new deal but the forward turned it down.
As the player is under the age of 24, he was too young to move on a Bosman free transfer and in such cases an independent tribunal decides a compensation fee. However, the two clubs have now reached a settlement
After being released by Aston Villa, Forrester joined Brentford in August 2011 and made 68 appearances for the London club, scoring 11 goals.
Injury has limited him to just two appearances for Doncaster, both as a substitute.
Forrester struggled with an ankle injury since joining Rovers but after recovering from that problem last month he now faces what could be another six weeks on the sidelines after tearing a quad muscle.