Ipswich striker David McGoldrick will play the last game of his loan spell on Saturday, but will return permanently in June if the club avoid relegation.
Town could only sign the 25-year-old for 93 days as he had already been on a long-term loan to Coventry this season.
"We could've waited until February to try and loan him but he might have gone elsewhere," Ipswich boss Mick McCarthy told BBC Radio Suffolk.
"Now we've agreed to sign him when his Nottingham Forest contract expires."
McGoldrick scored 18 goals in 25 games while on loan at Coventry in League One earlier this season and since joining the Tractor Boys has netted four times in 12 league appearances.
He scored twice in a 3-0 defeat of Leeds United last Saturday, but this weekend's game against Derby will be the final match of his loan spell.
Meanwhile, Jay Tabb has extended his loan deal from Reading until the end of the season, although the Royals have maintained a 24-hour recall clause.
The midfielder, 28, has started three times and made one substitute appearance since joining the club on loan last month.
Ipswich have also announced that five youngsters will not be offered new contracts for next season.
Ronan Murray, Cormac Burke, Joe Whight, Jack Ainsley and Gunnar Thorsteinsson will all be released when their deals at Portman Road expire in the summer.