Ipswich boss Mick McCarthy has offered a short-term contract to former West Ham midfielder Jack Collison.
Chairman Fawaz Al Hasawi has vowed to strengthen Nottingham Forest year on year, with loan arrivals possible.
The Turkey midfielder is currently on loan with the Bundesliga club from Real Madrid until the summer, but agreement has been reached on a pre-determined fee for the move to become permanent.
Dortmund have now decided to complete the permanent transfer of the former Liverpool man, who will not now be returning to Spain in the summer.
"We informed the Real Madrid management of our intentions to take the option on him during our Champions League quarter-final tie with them," general manager Hans-Joachim Watzke said to the Ruhr Nachrichten newspaper.
Sahin, who was born close to Dortmund in Ludenscheid, has not missed a single Bundesliga game for the club this season, establishing himself at the heart of the midfield in his second spell for the Westphalians.